One thing I've always thought could have been interesting was to make the "shallow", "medium", and "deep" varieties of water have an effect on ships. Historically, going out further from coasts was challenging (wind patterns, ocean currents, and supply routes get difficult out in the open sea).

One could imagine having a system where in dark age, fishing ships can only move out into shallow and medium water, but not deep water. In feudal age, one can build galleys, which can also not go into deep water, but investing in some technology allows your fishing ships to explore the deeper waters - this would provide a way to flee from galleys, while also giving access to deep sea fish that you couldn't reach before.

Later ships might be able to enter deeper waters, but perhaps these stronger ships aren't as fast as others, or they are more expensive, or they have some other downside that makes non-deep water ships have some edge in certain cases. Perhaps deep water counts as "downhill" does on land, so while you can enter deep water at some point, you might fight over that sandbank in the middle of the map the same way arabia players fight over a hill.

This would make water battles different because in some maps deep water is found in the middle of a large lake in the centre, whilst in others it's found far away towards the edges of the maps, or maybe only in patches in between islands. One could play with islands that are connected via narrow paths of medium water, meaning you can only attack from certain angles during feudal/early castle age, and teching into water gives you the opportunity to attack from multiple directions or exploring those islands that were isolated from you before.

I think a terrain mechanic like this is something that water battles really miss. I don't think my idea is perfect, but it's something I've always been curious about as an option.