It took me a little while to figure out what I was really doing and what my precise goal was. The tutorial helped, but I think a very simple first level (just catching one UFO) that purely teaches a player how to play and what goal to reach might be better. On that same note, figuring out each fish ability was a lot of trial & error. I'm a fan of trickling down new mechanics personally; start a player off with 1-2 types of fish (with more standard abilities), then add new types per level. Keeps it fresh.
However, after getting the hang of it, it was perfectly doable. Not too easy, but not too difficult to the point of frustration. I'd say the balance is alright. Not knowing what fish did what, did make me feel like I was just randomly clicking things at first; I'd still prefer the game to slowly introduce them one by one, it would make me feel more like I knew what I was doing from the get-go.