Chevriel, I suggest you work on developing the history of this race, where it came from, which side it's on (good or evil), what role it plays or will play, etc. Well, you get the idea.
And finally, a few minor criticisms regarding the creatures:
1. Isn't the 1st-level poison too powerful? In that case, in terms of usefulness in battle, they could be compared to stalkers, IMHO, since they will significantly slow down strong creatures. Maybe make it a regular poison or come up with something else?
2. Will the attack of all siren creatures be an active ability, or will it always be? To avoid imbalance, the damage should be distributed equally among all creatures, IMHO.
3. At level 4, IMHO, there is an imbalance in the upgrades. Sea voyages don't happen very often, so the Sea Wolf will be chosen less often. And I think an increase in movement by 25% would be more than enough, because negating losses in whirlpools is too powerful.
4. At level 7, there is an ability to sink a ship. I assume that only empty ships can be sunk?
P.S. Lock, thanks for not forgetting about that infamous worm race!