Lots of marine 340's built back in the day; mostly hooked up to Volvo Penta outdrives & called "Super Bee II". Also 318's (SB I) & 360's (SB III).
They were also put in all kinds of straight shaft or v-drive inboards, mostly competition-type ski boats.
If the block you're lookng at was used in salt water & used an "open" cooling system (sucked up outside water, circulated it, & spit it back out) I would avoid it because of the potential for rust/corrosion issues.
I've personally owned 2 marine blocks - 1 a 413, the other a Buick v-6 - that rusted thru the cylinder walls from the water jackets.