Mechanics: Their specialties are simple. They don't slip on water puddles, but they need to drink water (or spritz themselves with some mist) every several minutes or they'll start to "dry out". Drying out increases vulnerability to brute damage, and if untreated for a long time, inflicts brute and oxygen damage until death or rehydration. As with most other species, they have a unique language.
Appearance: This is the fun part. They can look like pretty much anything. Catfish? Salamanders? Sharks? Mermen? Kappa? The plausible variety of facial features and interesting colours an amphibious species could have is endless.
Some images, for inspiration:
Honestly, I love this idea so much that I'd code and sprite it myself if I weren't busy with so many other projects. So here ya go, forumites. Do feel free to drop suggestions/ideas down below.