That is totally fine. ¨Poder¨ means both ¨to be able to¨ and ¨can.¨
For me, also. It actually led into a bit of a discussion in our house because I thought it was "I can't drink without water." which is, after all, a bit odd. However, someone said they thought you could have "agua con gas" or "agua sin gas," basically bubbly water or flat water, and wondered if that might have something to do with it? Lots of people prefer water with bubbles, or dislike plain flat water. Turns out, of course, that it wasn't anything to do with carbonation. So, while I got the sentence wrong - the computer voice is tough for me - I managed to grow my vocab and understanding a little in process! The road less taken and all that. :)
Actually, the way I understand it, correct grammar would be ¿Eres un pez? Why is this? Apparently, in Spanish, they have (at least) two words for "fish": "pescado", and "pez". The difference is that "pescado" generally means a fish that is out of water or that has been caught, while "pez" means a fish that is swimming in water.