I think Age of Empires II could make water maps significantly more accessible by reducing the movement speed of all ships by 35%.
One of the reasons many players avoid water maps is that they have to manage both land and sea at the same time. Fast-moving ships make this even harder, because naval battles develop very quickly and fleets can move between different parts of the map in seconds.
Slower ships would make naval gameplay easier to follow and easier to manage, giving players more time to react and make decisions. It would also make naval combat more positional: players would need to think carefully about where they place their fleet instead of constantly chasing and repositioning.
It could also reduce the snowball effect. After losing a fleet, the opponent would need more time to reach every fishing area and coastline, giving the losing player a better chance to rebuild and fight back.
There is also a realism and immersion argument for this change. Large warships currently move almost like cavalry units, which can feel rather unnatural. A large warship should have a stronger sense of weight, momentum and limited maneuverability. Slower movement would make naval combat feel more believable as well as more strategic.
The goal is not simply to make ships slower. It is to make water maps less mechanically demanding and more playable for the average player, while making naval strategy more important than constant movement and ship spam.
I believe a 35% speed reduction could make water maps much more approachable and encourage more players to actually play them.