If humanity survived...
I'm going to assume the most basic technologies, since the thought of being able to extract groundwater or desalinate (mechanically) the ocean would be unlikely. This is just elaborating from depperm's answer, and adding some more suggestions.
Your folks on the few boats should have a little bit of water in the first days. My boat has a 220 gallons of freshwater on board, though I doubt she'd survive what you're describing, or that I would be able to get to it. Add to our boats any salvageable water containers (?) that have enough boyancy or air to float. There will be a LOT of flotsam: I'm swimming to the nearest semi-capsized or afloat mega-yacht.
A solar still, as pictured below, was available to air force pilots, if I remember correctly. At any rate, some tubing and plastic means that on a hot day, you can get pure water from evaporation.
This is tricky, but not impossible; on high seas, this might also imply rough weather, though. As mentioned in one of the other answers / comments, in a pinch it can be drinkable if you have vessals large enough to store it.
You can collect the morning dew easily with any (clean) bed linens, plastic or flax, by stretching it an angle from your boats or whatever you're clinging to. Much of the dew will drain downward into your container or you can squeeze the linens to get it before the sun does. The sides of our tents, even in the desert, would be soaked in the mornings with dew, so whatever fabric that was, go for it.
With salvaged water supplies, you can help (if you can't boil) by adding an alcoholic beverage to it. As an added bonus, if you don't go overboard, a moderate consumption of beer can be hydrating. I can confirm from ..um.. research.. that unopened beer cans float.
You will have to answer a lot of problems, not the least of which includes food, humanity, and also: where did 300m+ water come from?!