Interesting idea.
But I would go the opposite route. Maybe nerf fish booming a bit, not too much though.
But give the option to add some land eco in dark age. There are various ways to do so - it should be comparably expensive and limited.
This could even add some more strategic depth to non-water maps, allowing people to invest into eco early but with the caviat of heavily delayed uptimes.
A loom like tech for fishing ships sounds interesting. About upgrading each ship separately you have a good point there. The boarding ship sounds crazy but might be fun lol. About the Scout ship i don’t get the idea very much, idk. I guess in water you don’t scout much and if necessary you use a fishing ship.
If a transport ship dies then the units get on a boat . . then what’s the point on killing the transport ship? the opponent will get the units to the destination anyway, you would need to kill the ship twice. Maybe i’m missunderstanding.