In my headcanon, there’s a few different kinds of fluffies commonly called “seafluffies” or “aquafluffies”.
One is formally called a fishfluffy and it’s exactly what it says on the tin; a fish-like, water-breathing fluffy.
These would die in minutes if out of the water.
There are a few different variants tailored after different types of fish, but since they’re still fluffies, none of them are carnivorous even if the animal they’re based off of is.
Most fishfluffies are based off of goldfish, koi, or livebearers (guppies, mollies, platies, etc.).
One notable variant of fishfluffy is based off of seahorses, and is usually just called a seafluffy. These are notable in that the stallions, not the mares, are the ones that give birth.
The non-fish seafluffies are all variants of a more mammalian type.
Some of these are sea lion/seal-like, others dugong/manatee-like, and there’s also the occasional walrus-like or orca/dolphin-like variants.
The sea lion type (and the walrus type) can move about on land fairly well and often comes out of the water to rest.
The seal type can move about on land too, but not as well. It also will come out of the water to rest.
The manatee/dugong type can somewhat move on land, but not very far, and only does so if it spots food (especially spaghetti, of course).
The dolphin/orca type can’t move on land at all.
Being fluffies, these get beached much more often than the animals they resemble, leading to a slow and painful death for the fluffy due to either dehydration, overheating, or even being crushed by its own weight (NOTE: They’re heavy compared to normal fluffies, but they’re still not heavy or strong enough to harm humans or other animals).
Then there is the merfluffy, a seafluffy based on the mythical hippokampos creature. This type has the front half of a normal fluffy with the back half of a fishfluffy.
Like mammalian seafluffies, merfluffies breathe air. But due to their fish-like tails, merfluffies are in danger of drying out if they spend too long out of the water.