Yep, agree with the wildlife details, it adds value to unique locations you're sailing.
Seagulls when getting closer to shore, penguins when in artic area's, ...
Also my all time favorite game use to be Uncharted waters, where you could go out and explore exotic events/species.
I still remember discovering the dodo in Madagascar and getting a huge sum of gold for it.
Sailing deep into the Amazon river for Mercator and mapping out the area.
And the ability to customize the interior of your ship. (storage size, number of space for sailors, supply size, armory, medic post, kitchen,...)
It was originally planned for East India Company game by Nitro, but it didn't make it to the final game.
this would make your ship unique and you'll feel more attached to it.
Maybe this is a bit too much to implement on the ship models.