Fuel vent, adding via pump nozzle or jug

Boat not car.  When it was new in 2019, in the hottest part of the season there would be times that at the 30-35 gallon mark it wouldn't take any more (44 gallon tank).  Fall and spring, never an issue.  By this year in the hottest month it's really fussy.  Today we started with about 10 gallons in the tank and I couldn't get more than 3 gallons in it with a lot of spitting out.  This is with holding the fuel nozzle (typical gas station type) cracked just open for flow.  3 gallons took 10 minutes at that rate.  I know I need to get to the vent but I don't know how accessible it's going to be.  If I take the boat out of the water and try to fill it with the race car 5 gallon jug do you think it will fill any better?  Will it help to put a long hose on it to get further down into the tank?

You can't put fuel in the tank w/o venting air out.  If the vent is the issue, no amount of cleverness in the nozzle length or fuel source (can or nozzle) is going to fix it.

Figure out your vent issue.

Can you remove the cover on the atmosphere end of the vent line? Sometime bugs make nest in them and restrict flow. 

I'd try putting a hand held vac on the outlet of the vent line and try to draw air through with the fill cap off.