Here are some differences I've run across between Merc and GM crate engines:

Marine cam with low overlap, and wide lobe centers (pushes torque peak to higher rpm, while preventing reversion).

Stainless steel head gaskets

Brass freeze plug (some crate engines do have these)

Lower compression pistons (generally 9.4:1 or less)

Inconel exhaust valves to stand up to the constant high exhaust temperatures

Hard anodizing on aluminum heads to prevent corrosion

Better quality block paint to prevent corrossion

Bigger oil pan

Marine distributor

Marine carburetor or fuel injection

Full floating piston wrist pins (high performance engines)

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I'm sure there are a bunch of other things, too (valve springs, retainers, etc). It doesn't mean you couldn't start with a crate engine, but it would be best to make a few changes before pressing it into marine service.

Michael

At least as of last year, the mouse motors (small blocks) and the lower performance big blocks did not have Inconel exhaust valves. I am not sure if they were even HD rated. Mercury's price for them was something like $8-9 apiece. Way too cheap for Inconel.

IMO I think a 6.2 (320hp) with a B1 outdrive should do the trick. I think they can be had with closed cooling if you are running in salt. Maybe a little pricey, but reliable, warrantied power and an engine that should last a long time.