A relatively new addition to the .io crowd, Deeeep.io (4 e's) puts you in an ocean environment where you play as one of many animal varieties. Similar to Diep.io, there's an upgrade tree, as gaining enough points allows you to play stronger animals. Different animal types have special abilities that makes each one somewhat unique to play, increasing the game's replay value.
In another interesting move, the small pellets you can collect to increase your EXP are grouped differently in separate parts of the arena -- meaning you can go to an area with larger ones, but doing so risks running into stronger animals who had the same idea.
Check out Deeeep.io on its official website.