Is there such thing as a marine 340, I found an ad for a marine 340 and was wondering if boats ever came with 340's. And would the motor be the same except the manifolds?

Yes there were marine 340's...usable?? well only if they were fresh watercooled ..closed system with antifreeze...and if they were standard rotation..Left hand..stern drive engines were standard rotation ...single engine inboards were rt hand (backwards) and on twins usually the port or left engine was standard..

so what is the best way to tell if the motor is usable?

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.

I would pull the freeze plugs out and look for excessive rust! Pull a valve cover, If the inside is clean it might be std with little wear. Most boats are used very little in comparison to cars, It might be the best you'll ever find. The cam will be different if it is rev. rotation and the nurl on the crank is backwards as well as the dist. is designed to turn backwards. I had a boat with a 318 V drive years ago, It had been terribly neglected but the motor cleaned up fine. I cut the shaft on a big block electronic ign. dist so as to use in the boat. Good Luck!