Reasons for loss of water pressure?
Ones I've had to deal with:
1) Blocked intake, usually mud, from running too shallow
or mud wasp building a nest when boat stored outside
2) Worn impeller, blade tips/edges abraded by silt or sand
3) Impeller detached from metal core
4) Impeller housing seals no longer seal
5) Bad seal where water tube enters block
6) Blocked water jacket
7) Blocked or frozen thermostat
8) Blocked feed to pisser tube
and
9) The worst one, sheared drive shaft just above the impeller housing
One other thing, if I take the time to drop the lower unit
and I can tell the impeller hasn't been serviced in a long while
I don't just replace the impeller, I replace the housing and all seals.
The kit isn't that much more in price, than just the impeller.