First and foremost: sailing within sight of the shore is generally not a huge challenge. The real problems arise when you sail far away from shore.
Anything that aids with navigation
One of the biggest problems that faced the sailing world was that they didn't have GPS. The closest things were the stars and the sun, but the stars are only useful if you know the constellations (and when the weather is clear, and at night) -- so sailors from the northern hemisphere became lost if they sailed south, and vice-versa -- and the sun is only helpful if you have a good clock, which is an obstacle in determining your East-West position (aka longitude). This last is actually a huge problem that loomed over all seafaring nations for a long time, and it's had major impacts on history.
There's a brief non-fiction book by Dava Sobel called Longitude, which lays all this out in detail. Most of this section of my answer is taken from (my memory of) that. The subtitle is "The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time", and every word in that is 100% accurate. The problem was to accurately measure longitude by building a seaworthy clock.
You can't figure your longitude from the sun's position if you don't know the correct time. The problem is that clocks of the day were pretty bad; they lost many seconds or even minutes each day. So, you couldn't just synchronize watches on shore, because they'd be inaccurate before long and thus foul your navigation. Why?
Clocks of the day were either spring-wound or pendulum-driven. Pendulums do not fare well on ships that roll with the waves. Springs don't hold up well under the humid, salty air at sea. These were insurmountable engineering problems. The best minds of the Western world dedicated themselves to this task for centuries, with little progress to show for it. Kings set gargantuan bounties for anyone who could demonstrate a working solution.
IIRC, the guy who eventually solved this was a self-taught inventor who constructed a mechanical clock that was both tightly-sealed against the environment, and used a couple of redundant systems to counteract each others' predictable drift. I think it was a little bigger than a microwave oven, made of wood and metal, but was sturdy enough to go to sea. His solution was exactly what everyone needed, but the Crown stiffed him. Read the book, it's great.
Umberto Eco wrote a wonderful novel called The Island of the Day Before, which talks about some real crazy things Europeans did to try to solve the longitude problem. One theory that stuck out was the belief that there was some kind of sympathetic link between a weapon and the wounds it inflicts. So the thinking went: if you cut yourself with a knife, and then later the knife is heated, your wound will hurt, no matter how far you are from the knife. So, one experiment in the book is that a sailing ship would take aboard a dog that had been cut with a knife. Over the course of the voyage, they would deliberately prevent the wound from closing and healing (which is monstrous). Meanwhile, back on the shore, they would heat the knife every day at precisely noon, the idea being that the sailors would know from the dog's yelping that it was noon, which then allowed them to figure out their longitude from the sun's position1.
So, as far as magic: anything that can be used to reliably tell the time, even just once per day, would have been an absolute godsend. The caster would likely be the most highly paid person on the ship, even moreso than the captain or navigator, and all they'd need to do is report the exact time.
Or you could short-circuit the complicated navigational reckoning and just have the caster know the ship's true position. It would have been revolutionary. The political history of Europe would be different, because fortunes were lost due to navigational mishaps.
Anything that allows them to grow fresh fruit
Scurvy killed about half of all sailors during the Age of Sail. If you went to sea, you probably died from scurvy. Here's a pretty good summary of how bad it was:
Scurvy killed more than two million sailors between the time of Columbus's transatlantic voyage and the rise of steam engines in the mid-19th century. The problem was so common that shipowners and governments assumed a 50% death rate from scurvy for their sailors on any major voyage
-- Science History's Age of Scurvy
Scurvy is what happens when your body runs out of Vitamin C. Many parts of the human body need C, and when you don't get it, each fails at a different rate and in a different way. The results are bad. It's painful, and the brain also stops working right, so you also go kind of crazy. There is some speculation that famous sea monsters like the kraken, mermaids, and the like are actually based on sincere accounts from sailors who were suffering from scurvy. IIRC, it takes 3-4 weeks for the first symptoms of scurvy to appear. Here's a list of selected symptoms from Wikipedia:
- feeling tired and sore arms and legs
- gum disease
- bleeding from the skin
- poor wound healing
- personality changes
(That last item is a clever euphemism for being so delirious every waking moment as to be "bat-shit crazy.")
Left untreated, scurvy is always fatal.
The cure for scurvy has been discovered and then lost again at least 7 times in history. The final time it was discovered, it was the result of the first major experiment carried out under what modern society would consider "science": with a hypothesis, an experimental design, control and experimental groups, etc. And even then they almost bungled it -- and Science with it -- because after they confirmed that Vitamin C (from lime juice, IIRC) is how you keep scurvy away, the process they used to mass-produce C supplies for the Royal Navy used copper tubing, which denatured the C, rendering it worthless. This caused them to briefly reject not only their conclusion but the new-fangled "scientific" methodology they'd used to reach it.
It is no exaggeration to say that scurvy didn't just kill more sailors than all other dangers combined, it almost killed the enterprise of modern science itself.
So, magically: anything that helps you grow fresh fruit at sea. That could be the classic rain-maker, or it could be the ability to simply grow a healthy plant from a seed without water & nutrients. Or I guess if you could just summon fruit. (If you can teleport supplies, that undermines a lot of the purpose of sailing -- not all, but a lot: just teleport the goods you're shipping, or the travelers or colonists or whatever.)
Last, but still pretty good, would be the ability to control the weather.
Most importantly here would be the ability to control the wind. Sailing ships can literally only go where the wind blows. So, it's not accurate to think of the Atlantic Ocean as a giant open space where people can sail anywhere they want. More accurately, it was a vast desert with only a couple of specific routes that could be traveled, those routes being the trade winds.
The trade winds are in specific places, and they blow in specific directions. And the wind isn't exactly guaranteed, either. Ships were lost with all hands because they got stuck in the doldrums and ran out of supplies before they could reach a port. Also, by traveling along known routes, you're an easy mark for pirates.
A ship that could make its own wind could sail in any direction and by any route it chose. That would be a terrific advantage in trade, war, and exploration. You could sail in a straight line at top speed the entire way.
You almost might not need good navigation if you could guarantee the wind will blow a specific direction for the entire voyage (which would literally be a major miracle even if it happened only one time in history). A ship that could do this reliably would be so fantastically fortunate it's almost hard to imagine the impacts.
Of course, control over other aspects of the weather would be enormously valuable as well. For one, summoning rain would help combat scurvy, because you could keep plants alive. Not even grow new ones, just keep the ones alive that you brought with you from shore.
Also, bad weather at sea was a big danger. Storms absolutely sank ships, even without making them crash into things. Masts can snap, vessels can capsize, and a ship can only carry so many spares (and carpenters). Bad weather also makes navigation hard, and once the storm is past, you have to reacquire your position (which is its own challenge). Being able to prevent storms won't help you get to your destination or even know where you are, but it does reduce the risk of a successful voyage turning into a catastrophe in just a few hours.
To the very best of my knowledge, these were the three greatest problems in the Age of Sail: not knowing where you are, not being able to go where and when you want, and probably dying painfully because all your organs (especially including your brain) independently shut down before you can get there.
There are probably countless ways to tackle each of those problems. Any magic that engages with even one of them will be an incalculable boon; anything that fails to mitigate at least one of them will fail to fundamentally alter a vessel's chances of success.
1 Minor clarification: the sailors would know that it was noon back home at that instant. From that, and the sun's apparent position, they could work out how far East they'd traveled.