Were the 68-71 models high compression like their auto counterparts, or was compression lower in the marine application?

Didn't even know they made 340 marine engines. I've seen a couple 318 marine engines but that's about it.

I'm going to say they had the same major parts as the car motors did
I had a friend who had a Chrysler inboard boat with one of those motors in it back in the mid 1970s
I've taken a lot of the early, pre 1972, stock 340 motors apart and none of them had the piston decks above the block decks like Mopar provided NHRA blueprint specs. for
I have built more than one "stock" 1971 and 1970 340 340 motors for NHRA racing, which using the NHRA specs allows the piston deck height to be up to +.050 above the block decks
When I check those motors before taking them apart they had around 9.6 to 1 or lower actual compression ratios

A lot of the 340 marine engines, maybe all, used 318 heads. Trying to find out how much compression Chrysler was able to get by running on raw water cooled inboards back then. Wondering how high I can make compression on a small block 5.9 marine engine with magnum heads without causing problems.

Cal I have had similar experience with the older 340s. If I remember right blueprint spec had the early pistons at .018 above the deck for auto engines. Actual was much closer to zero from the ones I have seen.

I had a 70 340 boat motor, it had low compression cast pistons, cast crank, even had open chamber 318 small port heads from the factory and reverse rotation crank in single engine application. I still have the 4bbl intake off it, small port iron single plane with a very dramatic carb pad angle. The block would have been the only thing worth keeping butt unfortunately it was cracked.

I have a 1975 Super Bee III 340 boat motor. It is standard rotation but I haven't checked the heads.
318 small port heads would make since beings that a boat motor needs low end torque and is pretty much done at 4500 rpm.

A little off topic, but related. Have any boat manufacturers used the Gen III hemi for power?

In my opinion marine engines are a bad choice unless they a very very cheap or have a bill of health from a reputable machine shop.

ANY marine engine that ran salt water (cooling water from what the boats floating in running thru the water jackets) is junk. The water will eat the cast iron away to the pistons. Water jacket will crumble away. Dads boat had two 400s, replaced with complete engines the installed fresh water like a car system. Salt water still cooled the water chamber and risors.