I think it would take a really big effort to make oceans realistic.
- how long can you hold your breath
- does underwater pressure exist so you cannot just dive straight to the bottom of the ocean?
- how long can you survive in icy/cold/warm water
- how long can you swim before you're so tired you sink to the bottom. Can you conserve energy by just threading water?
- storms, how frequent and which vessels do they make sink
- skills for seamanship in order to ride out storms, quality values for parts of the ship
- skills for navigation at open sea
- drifting around on logs, making makeshift rafts, skills for controlling a raft
- stats for different types of water vessels, canoes, rowing boats, galleys, fisher boats. What happens if you loose the oars on a rowing boat?
- stats for how much you can load a vessel, what makes a vessel sink, what if it catches fire if you meet a dragon that breathes fire on your boat?
- icebergs and iceflakes to sail on
- can a sea monster ram your ship and make it sink?
- can you make vessels sink by smashing them with an ax?
Probably when sea travel comes, there will be weird things like you can cross oceans by swimming all the way or riding a fish, or that a raft made of a log works as good as the best ship the money can buy, or that some leather suit is a perfect diving suit
And a lot of drowning probably happens lol.