I launch boat drive for about 15 minutes engine alarm goes off motor is hot. Shut down fire it back up after about 5 minutes cools down. Assume its a bad thermostsat so replace gets hot again. Ran into some sand at the end of last season so chnage sea pump still wont cool. Take water pump off motor looks good no leaks? Also had outdrive off this spring. Cant figure what problem is any suggestions?? Thanks Mark. Atleast still have 2 working.

When you changed the seawater pump were some of the vanes sheared off? Did you find all the pieces ?If not,it is commom for the pieces to lodge into the passages and block water flow,even with a new impeller.

No impeller was fully intact. That whats I was thinking. I really think it has something to do with the outdrive but dont have much knowlege on those.

when you ran into the sand last year how much sand, you could have sucked it into your block I know thats not what you would like to hear but you know how stuff happens , Good luck with her .

wasnt that much but always in the back of your mind.

Is it possible you reversed the hoses re-installing the pump? Had a friend do this. Had very little water out the pipes ...

dont think I did but will check. I think I will have to take this in to a mechanic.

If you can change the tstat and R&R a drive, you can do some simple diagnostics. With the boat in the water (better water supply than hose), pull off the inlet hose to the t-stat housing. Have someone start the engine while you watch. There "should" be a substantial flow of water. Shouldn't take more than 1-2 seconds for that test.
If there's not a strong flow of water, work backwards from there: backflush fluid coolers, sea water pump, water neck at transom assembly. You can stick a garden hose in the sea water pump "inlet" hose and see if the water flows strongly out the drive; this one you do out of the water BTW.
If you have good flow at the t-stat housing, take the exhaust/cooling hoses off at the t-stat housing and stick the garden hose in them. You want to confirm good flow through and out the manifolds.
If you can get down there, on each side of the block under the exhaust manifolds, are the block drain plugs. Pull those and flush water down the t-stat housing. It's not perfect, but it may help flush the block if it has a bunch of sand in it.
Good luck.