Some of the older marine carbs from Mercury did not have j tubes.

From Mr Dennis Moore's Book... (a personal favorite of mine) As well as the J-Tubes:
"Throttle shafts are grooved and slabbed to prevent fuel from leaking out of the throttle shafts if the accelerator pump is activated without the engine running, or if the engine is flooded. The grooved and slabbed throttle shafts direct the raw fuel back into the intake manifold where it is consumed by the engine."
For what it's worth, I run a BG Demon (non marine) carb on my BBC. I do hope not to blow up someday!

1) fuel pump vapor nipple
2) J tubes
3) teflon bushings on throttle shafts

I know those 3 three things are USCG mandatory

Final Answer
 

Holley Marine Carb We all know about J Tubes The Throttle shafts are grooved and slabbed to prevent fuel from leaking out of throttle shafts. The groved and slabbed throttle shafts direct the raw fuel back into the intake manifold. If a flood condition should happen.
Calibration; richer pump shot and mian metering circuit activated at lower rpm.

This is from Dennis Moore's book. Big-Block Chevy Marine Performance from HP Books. Section 1 The difference between Automotive and Marine Engines. Section 1 covers Electrical Systems , Fuel Systems, Fuel Pumps, Carburetors, Hoses, Crankcase Breathing, The book is a most for perfomance boaters doing there own worK