I stepped into the Tropicraft realm on my modded world and I found no mobs, zero, spawning in the water.
I searched around the world for a bit and swam underwater but I found no fish spawning. Other creatures such as Monkeys, Iguanas, frogs, parrots, etc. spawn on land in a proper manner but I couldn't find any sea creatures.

Tropicraft Version: 1.12.2 / 7.1.9.115
Forge Version: 1.12.2-14.23.5.2838

Do you have any other mods installed, particularly any that impact mob spawning?