Some good points already made. If you can get the heads off or a core plug out you can get a bender light (inspection light) around the outside of the cylinders and tell what sort of environment they've been in. Check also the direction of rotation as some boat motors are reverse rotation. That means they need another cam and distributor gear. The mod to the rear crank seal is to remove the grooves on the crank so it doesn't wind
oil out of the motor. Boat motors can be good if they're solid cylinders. They don't suck road dirt and keep pretty good bores and they get warmed up and run for hours at a time. Not like a car motor that might be doing 2 minute store runs. It's worth a look. Good hunting.
Cheers Nick