Gonna echo what others have said here about the turning rate-- I grabbed every turning upgrade I could get my hands on but still got outscaled (hah) by the size increase debuff to turning. On my 4th upgrade I didn't roll a turning rate upgrade as any of the three options and at that point the fish became pretty unmaneuverable.
That said I'm a sucker for this sort of endless upgrade system. I kept playing despite the problems because ultimately the dream of becoming a large fish and eating everything is a powerful one.