Why isnbt my engine pumping water

i just bought a 1989 chaparral

i just bought a 1989 chaparral sx2350. it has a 7.4 litre 454 chev engine with mercrusier bravo 1. before i bought it i got the guy to start it in the yard and it worked fine. then we took the boat home and accidentaly ran pretty warm water through it( through the garden hose). and then this morning we tried to start it and the alarm started going off (over heating alarm). we then replaced the impellor and it worked fine for the first 3 mins then the exhaust stoped spittinhg water out. and i don't know if this has anything thing to do with it but when the outdrive is up all the way then the exhaust bellows isn't long enought to reach the recieveing end ( near the to of the shaft). does anybody know what i need to replace?

What condition was the old imp

What condition was the old impeller in? Sometimes if the old impeller was brittle a piece or two might break off and lodge in the inlet tube and would slow the water flow.