I like how I actually know the context of those reasons because I am not-so-secretly the other person in the thread where the topic arose.

The Dex says they live in swamps so they can probably...at least somewhat hold their own as far as water is concerned? They don't learn Surf but they are supposedly based on sea slugs so it's not like large bodies of water are outright anathema to them or anything.

Let's go with, it'll manage to float but it'll kind of spread out into a big floating...pancake of sorts atop the water, which will reconstitute and slorp back into Goomy shape once you fish it out. :|b