Cracks could be in the heads, main bearing support webs, or anywhere in the water jackets. If this engine has spent time up north, it would have been winterized every year. If not done correctly, water could have froze and cracked a head or block. These are just some things marine engines are at risk for.
You say the coolant was
antifreeze/water mix, so if true it would have to have been a closed cooling system(fresh water cooled)
Have you actually seen this engine? If it has a closed cooling system, somewhere mounted to the engine will be a large cylindrical heat exchanger with lots of hoses going to it and a radiator cap on top of it.
As far as rotation goes, I don't know about FE's , but I do know that 351W's require a different crankshaft for reverse rotation!!
(very expensive!, I bought a NEW reverse base engine from Ford for $200 LESS than just the crank!!)
All that being said, I think his price is way too high !! This is a 35 Year old BOAT engine with how many hrs???
See if he'll give you the phone # of the guy he sold the other engine to, call him and ask what he found when he rebuilt his.
Good Luck!
Ted