Jak and Daxter: The Precursor Legacy

No, you can't kill it. Its the way the game says "you can't go past this point". If you don't turn back you get eaten and start from the last checkpoint. Get on land or some kind of platform FAST.

Yeah I tried beating that annoying fish but you can't beat it but it would be cool if you could but another fish would just eat you

The lurker fish cannot be "beaten". If you are not supposed to be in a certain area of water (the game describes it as any water that isn't shallow) you will hear the sound of a heart beating, getting faster the longer you stay in deep water, which is a sort of timer. Either get on land/shallow water fast, or you will restart at the previous checkpoint after being gobbled up.