The world is over 99% water with only very small bastions of solid land or ice covered land. The humans who live on this world are very capable swimmers and divers who can hold their breath for a very long time. In general, the people are smart however they are held back from an industrial revolution due to their lack of metal, fuels, and simply because their population is limited by the size of the island. Technological progress has stagnated or for a couple hundred years while theoretical work including math, physics, and philosophy have continued to advance. For example, scholars on the island have a firm grasp of calculus and Newtonian physics but have dismissed ideas of making a steam engine as it would be too expensive and impractical (which is true, they simply don't have the metal or the need).
The people have wood and other agricultural/biological resources such as fibers and cloth for sails. Metallurgically, the people have somewhat advanced techniques but due to the small size of their island, they don't have a lot of metal. Most of the metal they have is bronze however they also have a large amount of meteoric aluminum. At great cost, artisans can create mechanical clocks and devices but these are reserved for the island's elite.
Additionally, the people have quite a good grasp on optics as the nature of their island allows easy glass-manufacturing. Plants with large reflective leaves allow them to make cheap mirrors and reflectors for long range signaling or concentrating sunlight to cook.
Due to being a water-planet, the land that the humans inhabit has basically no natively-evolved creatures on it and it is very safe. On the other hand, the sea is very dangerous. All varieties of massive fish, giant tentacled beasts, or venomous jellyfish can be found there and occasionally attack ships. The largest creatures are big, think kraken-attacking-pirate-ship big.
The people, besides needing to travel between their tiny bastions of land, rely on the sea for a large amount of their resources. Fishing is very popular along with trapping (like lobster traps) and diving to harvest underwater plants and resources. Additionally, there is a high cultural drive towards exploration as finding new lands to support the growing human population is seen as important.
What features might a sailing ship include which helps the crew fight against frequent monster attacks or make their travels across the dangerous ocean safer?
- Sharpened wooden spikes lining the outer edge of the ship's hull to combat giant tentacle attacks
- Barrels full of dead fish that the crew can throw overboard to distract any predators
- Listening posts or portholes below the waterline of ships where specially trained crew members can listen or watch for incoming monsters