Gasoline Fumes from Carburetor

If you can smell it, it can blow. However, the fumes lasting throws me off. I'd think leak because of that. There are carb guys and then there are carb guys. I'd suggest having a different one look at it. The carb itself may be leaking, or the fuel pump, but I'm thinking the carb especially since it was just rebuilt.

I am not a mechanic, but heavy gasoline fumes from a running, properly aspirated engine doesn't seem normal. Why do you say the source is "clearly the carb"? What evidence do you see?. It would seem that if you have run the carb dry and turned off the fuel supply, but the odor is still there when you check, the carb isn't the source.

Also check that the tank vent line is secure?

Both parts of this sentence are big red flags for me. If the gasket is not applied cleanly that could be a source of the leak. Any amount leaking there would quickly evaporate/ burn off while running, leaving no residue, and the amount may be extremely minor at rest. Check that area closely while running, as any leakage may be imperceptible when not running.

Bingo. What you describe could be caused by an extremely slight amount of seapage. Try running a clean thin paper towel along the edge of the gasket right after running. It may show on that. That carb may need to be pulled, the surface thoroughly scraped and cleaned and a new gasket seated.

P.S. I probably don't need to tell you this but I will. This is a very dangerous situation. No cruising. Run it only to check and keep a fire extinguisher close at hand when checking this.

Older Q-jets had problems with their floats that may not be caught by a mechanic unfamiliar with their many nuances. The old style floats were made of foam with a coating to keep gas from soaking in and making them heavier which will cause them to sink even in a full bowl and keep the metering rods open. That will overfill the float bowl and excess is dumped either out the side or into one of the throats. Older foam ones often scraped against the sides of the bowl chafing through the coating and they're finished. Newer ones have soldered brass floats that fixed this problem, before they came out I used to make my own but it was a royal pain.

I second the suggestions of a second opinion , Q-jets are probably the most difficult carb to master and the number of mechanics around nowadays who are qualified to tear into one has to be dwindling. Holley's, Edelbrocks and Webers are childs play compared to Q-jets, make sure whoever pops the top on one truly knows what he's doing! Failing finding one who is truly familiar with them I would buy a rebuilt one from a reputable marine supplier and send yours off and have it rebuilt for a spare.

77,
How does the boat start? Does it take more cranking to start since the carb was worked on? Black smoke out the exhaust when starting?
Where are the fuel tanks, aft or mid?