Re: 2000 15 nissan 2 stroke
Having the precise model/serial would help; Year is unimportant for service issues.
If you installed only the impeller, I would consider installing a full wp kit instead. The kit includes the cup and wear plate, which are close-tolerance items. If they are worn, a new impeller will likely suffer premature wear.
Is the motor in fact overheating, or is it just a matter that the pee tube isn't flowing? You can see if the telltale is clogged by pushing a stiff wire into it while the motor is running. If there is salt or silt accumulation there, that will likely clear it.
Make sure you reinstalled the impeller drive key correctly, or the impeller won't pump at all. Also be sure that you re-aligned the water tube with the water pump when reassembling. If not, the pump is merely pushing water up into the exhaust housing.
