I have a pair of 502EFI's. After running 50 miles @ about 3200 RPM and problem free I came off plane and starboard engine promptly died. It would fire but erratically and unusable to dock with. I did get it in gear running and got on plane and ran 3200 RPM for a few more minutes seemingly running OK. Throttle position same as before. Still won't idle. Plugs smelled of gas big time. Boat behind me smelled gas strongly. Plugs pretty black and distributor cap less than real good. Changed plugs and distributor cap. No difference. Tach jumping erratically almost as if electrical something going on.

Marina shop guys said really bad smelling gasoline and said try a load of Sea Foam before tearing down to look at injectors. Gas mostly from 2016.

Any suggestions ?

Scan the engine for codes, year ++ old gas needs to go.......could be IAC

Don't use any snake oil (seafoam or other)

Could use a serial number, is this a VST or cool fuel?
In any case, sounding like MAP has gone south or fuel pressure issue. Need to check your fuel pressure at the rail. Dump the filter into a plastic bag and see if you see anything in there but clean gas. If crud made it to the fuel pressure regulator it can clog it up and cause pressure to increase above spec

Motors are VST. I installed extra fine filters in front of VST years ago. No VST issues. Sea Foam is snake oil ? I thought just about everybody used Sea Foam to keep gas good on lay up.

No problems with VST, I removed the one from my working motor prior to rebuild, went to cool fuel. The regulator has a screen (item 17) if it gets clogged will increase pressure on the rail and can cause over fueling. The MAP sensor can hurt along with if the vacuum line came loose.

Genuine Mercury & Mercruiser parts. VAPOR SEPARATOR TANK & FUEL RAIL, SN & Up (See Note)

Need to measure pressure at the rail.

I don't put this kind of stuff in my gas tank, Seafoam is made of
PALE OIL 40-60%
NAPTHA 25-35%
IPA 10-20% Isopropyl Alcohol

Nor do I use Stabil in anything but gas that will sit more then a year
Even E10 will last a year.

Yup, check that screen if fuel psi is too high on the rail !
and if fuel psi is okay but running rich can be caused by ruptured diaphragm in mechanical pump or in the VST, either will have fuel go out the 'vent line' and pour into the intake. Same with if the float / needle and seat if get's stuck open.
Genuine Mercury & Mercruiser parts. VAPOR SEPARATOR TANK, SN & Up (See Note)

Anyway, as you can see, many different things can cause this.

Check first:
vacuum lines to fuel psi regulator.
Then fuel psi itself (gauge is relatively cheap -(most autopart stores have them for under $40)
Then condition of rest of ignition system: rotor and wires (since you replaced just cap and plugs)
Then get it scanned with emphasis on IAC and MAP
Then back to OSO
Then compression test if not done earlier

SB nailed it. I'd lay high odds this is the problem. It could also be the mechanical fuel pump diaphragm is ruptured and pouring into the intake manifold.

If the tach just started jumping when the problem occurred I'd be looking at the ignition system. I recently had a similar problem on a 502, that supposedly had all the components already replaced. Put in a new distributor and fixed the problem.

The tach "jumping around" is probably back firing.

When you pulled the plugs are you sure there was no water on them? I would pull one plug at a time and turn the engine over (make sure you pull the coil wire or take the lanyard off so the engine doesnt try to start) look for water coming out the spark hole (shouldnt be any).

Water in the cylinder will also cause a rich condition.

Just something else to check, but I think the other guys are right on with fuel problem.

Good luck