New impeller in July (didnt replace housing just impeller) running down lake at 35 mph. Exhaust sound changed dramatically and temp started to go above 160 so knew something was up. Cut engine. Towed home. Remove seawater pump. Vanes on impeller look new, but shaft just spins inside of impeller. what could have caused this??

When you reinstalled, did you lay down the vanes the same way old one came out?

From your comments, I need a bit more info

Can you post a pic?

Tried to post but got a BS note saying I cant post pics until Ive done 10 posts. Been on here for 10 years I guess I am have not been active enough. It looks like the impeller started to burn up from the inside out. Have never seen this happen, have always had vanes break off and burn up, never saw the inside get rounded out. Maybe the housing was messed up and caused the impeller to lock up the vanes. I did not install the vanes the same way as they came out on the new impeller, I thought I didnt matter that they flipped around to the correct orientation. I used car soap so the startup would be slick. worked fine for several boating outings until this past weekend.

Ya got 10 post now, it should work

If the shaft spins but the impellar doesn't, see if the keyway is still there. The keyway is what holds the impeller in place on the pump shaft.
Or are you saying the the metal the rubber is bonded too spins also and the rubber unbonded from the metal tube in the impellar ?

Looks like the keyway inside is rounded out/stripped