To be fair, the assets are really well done and (shocker!) could be used in an actual game! So in case somebody (including you) will want to make an underwater themed game - you’ve already done at least half of the work :D
But in case you want me to actually cavil, I’d say learning subpixeling is a good start to making your animations look cooler and smoother! (Truth is, you won’t need to re-do all your animations! Just add some inbetween frames and learn how to imitate <1px movement)