And park boat at pump so filling from high-side. Also on my Phoenix I used a drinking straw to clean the vent tube.

Had this happen on a Stratos 30 years ago. Dealer (fishing same tournament as me) said there was a “anti- back flow” device in the vent hose. (It came off the tank separate from fill hose) This is so that fuel won’t pour out if the boat flips.(?). He fixed it quick by shoving a long stick / rod / tool ?? down the hose and busting the float.
Could have been giving me a load of BS but it worked after his fix.
Boats may not have this anymore. Been a long time.

+3 this will very likely solve your problem.

Just curious, what does this do?

On mine it changes the angle of delivery just enough so the stop fill nozzle doesn't have as much back pressure. Thus the nozzle will keep running.
Mine is very finicky..

I seems to help if the nose of the boat is up hill . Just a slight incline will help

You are a very odd individual

^^^ this. My Pontiac Solstice has the same issue, and it's a huge pita.

How many times did you have to brush your teeth to get the taste out of your mouth?

Mine is the exact opposite. Only way I can get fuel on the water is to get two or three people to stand on the bow.

read this...it'll help you understand the sources of the problem and fixes... Boat Vent problem
Also, there IS usually a rollover valve that can get stuck if you fill the boat too full often. Usually if that's the issue, the bouncing of the trailer after pulling it on the highway for a few miles fixes the problem, but that's the last thing to check. None of this should be an expensive fix. Rollover valve

Thank you guys for responding, I haven't been out for two weeks now. I'll try the upside down fill method.

I believe I have a vent line, 1/2 hose running along top of tank but it disappears as it turns 90 degrees and up toward the fill cap and I would have to try to take the deck apart to look closer.

There is no visible vent at the fill cap, just a threaded cap about 2 in diameter.

There is one more hose to the pump bulb in front to the motor.

Boat is 2005 Nitro 750 with 115 hp Mercury so I don't think it has any fancy emission stuff.

If it's doing this it is most definitely not breathing right. Vent line issue. May be pinched or if it has the charcoal filter as someone said that could be clogged. Probably not blocked by a dirt dobber or a spider, gas fumes would prevent that.
I've fought this battle with pretty much every boat I've ever owned; at least a little, with built in tank or tanks. The Skeeter I have right now knocks heads with me at the pump with this issue. I've found that just pulling the pump nozzle about half way out resolves this most times.