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Cats are awesome too! There are many options, of course, but for the expansion (if our prayers are answered), they will still choose only one.
Now about the units. The idea of a poisonous Margol is indeed from Warcraft. If we make its damage 1 per Margol creature, it will be like an upgrade to damage, which is often found in Heroes 5, the only difference being that it lasts for several turns. Next, if the Kraken is unrealistic, I will remove the Kraken. Then, I will put Sea Elementals on the 4th level in the castle. We can even give them a special ability - a sea storm. The Hero, whose army includes these creatures, gains the ability, in map exploration mode, to cast a spell that pushes the enemy ship away, depending on the level of Chaos magic. I just need advice: would it be better if it worked across the entire map or only within a certain number of cells from the Hero? It is used as follows: if a large enemy army catches up at sea, it pushes them further away from you, and if a Hero with a small army is in a water castle, the storm pushes the enemy away from the shore.
Regarding the units, according to the general opinion, they will be as follows:
Level 1 - Margol
Level 2 - Triton
Level 3 - Undine
Level 4 - Pirate
Level 5 - Sea Elemental
Level 6 - Ichthyander
Level 7 - Sea Monster
As for the Siren's skills, it is indeed very weak damage, hitting all enemy monsters, the whole point is in accumulating the number of Sirens, which over time can overwhelm even a strong enemy army with their numbers.
And finally, the maps with the water castle will be predominantly sea-based, so I think the upgrade of the Pirate to a Sea Wolf is very successful. Thank you for the comments and suggestions.
Added 10 minutes ago
Again, a mistake, I wrote that the Elementals are level 4, but I made them level 5, but never mind, my own hand is the master...