For water map, an upgrade for transport ship

On water map, many civ have no chance to win.
If there is a quite expansive upgrade in castle age to increase a lot the HP and a bit the armor to transport ship every civ can win a game.
A land civ can win a game on island map with land fight.

You mean like Careening and Dry Dock?


Yes, but affect only transport ship

I understand your reasoning but it would make little sense that civilizations that are good with navy end up with the worse transports. Furthermore you ideally don’t want to wait too much to land since the more you wait the further away they get. Additionally some player feel too safe on their island and won’t get loom until later in the game, which really makes it good to attack early.