Personally, I prefer there to be a clear separation between “sea-fluffies” and “aqua-fluffies”—the former being mammalian and able to go on land, and the latter being more fishlike in that they have to stay in water at all times (Aquafluffies can have mammalian features and seafluffies can have fish features, of course, what matters more is whether they have lungs or gills). Whether or not the former can breathe as easily on land as on water is a toss-up, but I like them being somewhat amphibious, as it lets them interact with people more often.
For my personal favorites? Well, for seafluffies, Bad_Roomie’s take on them is hard to dislike in any way, though that’s the same for any of their designs imo. But in my eyes, it’s kind of a toss-up between Carpdime’s and SqueakyFriend’s. Carpdime has more examples than Squeakyfriend does, but both of them look exactly like what you’d want out of a cuddly aquatic mammal pet. While looking through these, I also found out that Squeakyfriend once drew a unique kind of seafluffy invented by PeppermintParchment, which is very neat and unique in that they have normal legs, but with fins attached to them instead of replacing them. I quite like these, but they don’t seem to be nearly as popular as ones with only fins.
For aquafluffies, I’d have to go with Rescue_9’s way of drawing them. The way they make the fur look like scales is great. A close second is FallenAngel’s interpretation of them—they make the more fishlike features look surprisingly natural on an animal with fur. Basically, Rescue_9’s design blends fluffy and fish features together, while FallenAngel’s is more of a really well-done Frankenstein’s Monster of the two.
Anyway, personal headcanon time! I view aquafluffies as being much cheaper than a regular fluffy and less bothersome to humans. They’re made to be calming to watch, much like regular pet fishes, but also to be entertaining as well, which is why they act like, well, fluffies.
Seafluffies, on the other hand, are a little cheaper than normal fluffies, with the required equipment for them averaging out the total costs between the two. They’re designed to be pets and companions to people who live in coastal areas, near lakes, or basically anywhere with bodies of water nearby, to cut down on fluffy pony drowning incidents while also being capable of existing outside of water. In my headcanon, some people will purchase seafluffies even when they don’t have access to water or places for them to swim for the same reason a non-abuser might want a pillowfluff—their lack of mobility on land makes them easier to keep track of and makes them less likely to get into trouble, without permanently crippling the fluffy.
This was a lot, but I hope that at least some of this helps!