In process of purchasing a boat w/ 500efi's w/ 200 hrs. All that is left is the engine survey. Here is what I have told them to do.
-sea trial w/ visual inspection/check oil
-scan for codes/verify hours etc.
-compression check
-gimble wiggle test

Anything else that is fairly easy to check?

Leakdown

Don't know of the room situation in the boat, but it would be great if you could get a valve cover pulled. One outside one on each motor. Just my 2 cents

Check for blowby at idle.
Check for moisture condensation under filler cap.
Hit manifolds and risers, (or headers), with heat gun checking for hot spots.

500 EFIs usually need new valve springs about 250 hours.

check drive oil for water, drain press & vacum test.....

as 88 fount said, a clyinder leakdown test is a good idea and at same time check plugs

All of the above and put scan tool on,check for fault histories,trouble codes and verify hours.If you get the SN# on both engines,have a Merc dealer run them on MercNet and find out histories while they were under warranty.Biggest problems w/ those engines were fuel pumps,valve springs and headers(just plum rotting out).Check propshafts for twists too.