We have a 1994 celebrity boat and we have changed the impeller but it is still overheating. Why??

Question

but it is still overheating at idel and when we take it out,

Answer 1

did you remember the impeller key?

thermostats/poppet valves would be next.

Answer 2

We need more information, like what engine and which impeller did you change, or have you checked the thermostat....etc.  Edit 7/29 1200h. That helps.  If the marina did the work, I'd first take it back to them, and ask what's wrong......If they comeback with a "new" problem, ask if they tested the motor after repairs, or why they didn't catch the first time.   It might be a simple repair and they may or may not take care of it......We guaranteed our work, but sometimes the customer wouldn't tell us about a "different problem" so we'd fix the complained of problem, only to later discover another major problem that the customer had known about but "forgot" to mention.  But, first give them the courtesy to check it out.