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.). Those based off of livebearers are the same size as microfluffies, earning them the nickname “aquamicros”.
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 more mammalian.
The first two types are very closely related, being based off of manatees and dugongs respectively.
These types can sometimes pull themselves up onto land, but not very far, and only do so if they spot food (especially spaghetti, of course).
Being fluffies, they 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).
Both types also have their mammary glands underneath their front flippers just like the animals they’re based on do. Naturally, the slang term for these fluffies’ mammaries is “pit-tits”.
Some fluffies of both types have hind flippers, while others (like real manatees and dugongs) do not.
The third type 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.