I see they sell a marine carb and normal carb? What's the difference in them?

I believe the main difference is they have j- tubes instead of straight vents and they might also set it up differently (jetting/ acc pump cam etc.) for marine use.

Sealed throttle shafts.
Most have "j" vent tubes, but there is no requirement to have "j" tubes in a marine application.

I always thought the "J" Tubes were to dump excess fuel back into the intake in the event of a sticking float ?

Yes but not a law to have them.

The whole topic seems to get a bit murky. I'll bet if you could manage to actually find all the info and interpret it there'd be differences between what the coast guard wants vs the insurance industry vs whoever else had their fingers in it. It'd likely get more confusing when figuring out if it applies to individuals or manufacturers or professional repairs vs your own repairs. Fuel leaks into the bilge seems to be a big no-no, so I'll bet j tubes are high on someone's list. I had to look into this once upon a time regarding some fuel lines, but that's a different story.

I had a set and besides the J tubes, they had "spring-loaded" bowl screws. Screw top--spring---screw bottom. I assume the spring was to absorb shock and keep the bowl screw from loosening and leaking fuel. The J tube is to dump overflow into carb instead of all over the place of course. Other than that, I never found anything any different with them. (the 750cfm in a marine form is 715cfm---supposedly that's how much cfm the J tube blocks)

Problem with "j" tubes is that unmetered fuel can get sucked out of the tubes and go right down the throttle bores.

Again.....NOT required by the USCG.

True. It's funny what you remember.

There was Speed Shop chain now defunct (can't remember--was Yellow building with brown stripes--anybody?) Any way, I went in and started talking about carbs. We spoke for 30 minutes, picked out 2 carbs, and he asked what I was going to put 2 850's on. I said, "a twin engine boat". Holy cow, you thought I had shot this guy's dog. He wasn't going to sell me these carbs because they weren't "marine" and I was crazy for even thinking about using these, and we went around and around until finally I just told him to keep the damn things, I'd order them somewhere else. I've NEVER run marine carbs, and it's been fine.

Super shops?