Question about a 2000 454 7.4 mpi mercruiser. Looking at a boat and the current owner sucked up sand and overheated the engine to 245. Boat mechanic checked the compressions and they came back between 165-175psi. The mechanic replaced the whole exhaust, rider gaskets, seawater pump etc. plus flushed the engine cooling system of remaining sand. Mechanic took it out and said it was running awesome. I guess I'm wondering if I should be worried about anything in the engine itself with the temperature it got too?

it might ! and it might not ! always hard to tell... head probably warped or twisted a couple thousands... heat sometimes gives more fatigue to some component, might shorten the engine life... but might not ! compression seems good... hard to tell but good luck anyways!

His mechanic or yours??????

A brief 10 seconds a 245* water temp might not hurt anything.
3 minutes at 245* could cause some serious damage that might not show up right away.
Head gasket might be close to blowing. There could also still be quite a bit of sand inside the engine block which could create some hot cylinders.