Tested on version 1.22.8 of the mod, with forge 1.20.1-47.2.17.

Here is a list of observed issues relating to creatures not despawning or otherwise occupying slots for their creature caps when they would not be expected to do so:

• Albino cachalot whales do not despawn. This is concerning because, even though they are rare, they quickly fill the water creature cap as players explore and spawn them in, since the cap is of just 5.
  One solution would be to let them despawn, just like the regular cachalots. Another could be to add a config for how often a cachalot spawns in as albino, so that players could disable the variant that way.
• As stated by the animal dictionary, catfish that have eaten something will not despawn. However, even catfish that have not been bucketed leave the water ambient cap when eating, leading to more catfish being allowed to spawn, which then go on to leave the cap themselves as soon as something small spawns in for them to eat. This can eventually fill a body of water (one players may not even be aware of) with catfish, causing huge amounts of lag. For smaller catfish, they can become persistent after eating items dropped by water mobs dying from beaching themselves. I believe that mobs becoming persistent so easily without player input is a huge issue.
  My suggestion is that the catfish despawn rules be changed, or a config be added, so that only catfish that have been bucketed become persistent.
• Crocodiles do not despawn under any circumstances, as far as I can tell, thus filling the water creature cap. Even imprinted crocodiles take a slot.
• Tamed mantis shrimp do not leave the water creature cap, so they prevent more water creatures from spawning.

In general, it seems that the water creature pool presents a lot of issues, it is concerning because it's already a very limited pool, so players miss out on things like even meeting a mimic octopus, which might just be my favorite mob of the mod. It does make me wonder if some of these mobs would not do better in the passive/creature pool, so that they spawn on chunk generation instead, as a more general solution.

This is already the case for the caiman and the alligator snapping turtle and it matches the behavior (but not the spawn group) of the crocodile never despawning. The crocodile behaves very much like the vanilla turtle, and so I would expect it to be in the same group. The catfish also seems too complex to be considered an "ambient" mob, although with tweaked despawn rules it should be fine.

As it currently stands, mobs in the water creature pool are also effectively unfarmable, as well. Mimic octopuses can be bred to farm ink (which kinda becomes a necessity since the water creature pool is almost always full), but their offspring despawn instantly once the player moves away, since they are classified as water creatures. Similarly, a bred mantis shrimp will despawn fairly easily unless tamed at birth. Not to mention that farming either of these almost instantly results in not having any more water creature spwans serverwide.

All in all, I just think that the water pools do not behave well with complex, involved mobs that present taming mechanics and the like. The cachalot is fine there, I guess, as long as it despawns like it should, but the other mobs, maybe not so much.