I love myself a strategy game that is equal parts simple and tricky. This also has a nice vibe, and I think the simple, blocky designs give an almost board game feel that I personally dig quite a bit.
Would definitely benefit from a tutorial. I was super confused at first, even having read the instructions on the page. I found that sometimes I was clicking a space instead of clicking the fish that I wanted to move, which was a bit annoying and added to my confusion. As others have said though, once you get through the initial learning curve, it really is quite fun! The fish that switches places with the humans proved to be an especially interesting mechanic.
I could see this being fleshed out a bit into something really top notch. Great work!