30724051
30724662 me

Another small note to consider is, and don't take this the wrong way, that spec. Biology of sea creatures inherently needs to go over a lot more evaluation and brainstorming than land animals, because you are dealing with what is essentially one large biome spread over the entire planet.
And once you add into that migratory species like whales, who travel a whole fucking lot (like holy shit, these fuckers can travel up to 19k km a year!) you really want as much input as you can get from other anons that might be interested in developing their own seas.

If we return to the magic blocking fur for example, this wouldn't really be something that is that important(the fur itself) if you are already developing a sizeable bludder layer. So it would make sense, from an evolutionary standpoint, to slowly phase it out. That is, of course, if the fur doesn't serve an important role in your survival when fighting other species. But even then, if most of your potential predators are not using magic based attacks aside from that one weird squid in your backyard, it would again be pretty redundant to keep the feature. It would make more sense for the species as a whole to simply abandon the area the squid is in (this is called predator pressure I believe) and find other breeding/feeding grounds.

Holy shit phoneposting is bad for keeping my train of thoughts coherent in a post, but what I am trying to say is, unless you want to make the area your sea creatures live in extremely limited, you have a lot to consider and want to get as much feedback as possible, so the ideas make at least some evolutionary sense on a grand scale. No need for magic repelling fur if only 1-2 species you might occasionally meet use magic based attacks.
Of course, there is a lot more angles to consider, perhaps the magic repel is just a side effect or something like that. Idk I am just rambling at this point, but you really activated something that was dormant for way too long in me. Too much fucking diplomacy and nothing else these days I tell ya what.