I bought my first boat several weeks ago.  It's an Xpress SV16 with a Yamaha 50 hp 2 stroke.

 The generous former owner left me with about 6 gallons in the 18 gallon tank.  so this morning I went to "feed the baby" some non ethenol goodness but as soon as I pulled the lever on the gas pump, it spewed back out at me.  Now knowing a little bit about basic physics tells me that the air has to escape to allow gas in the tank.  

The motor is a 2 stroke 50hp Yamaha so there are several hoses going into the tank.  

There is a big hose about the diameter of a golf ball from the filler hole,

a smaller hose  attached to what looks like an oil filter which then goes to a primer bulb,

and another smaller hose that runs towards the filler cap as well.  Not sure which one is the vent.  Has anyone else had this problem?  Maybe a picture would be better.

Most tanks there is a screw right in the middle of the gas cap that is the vent.  If that does not work post a pic. I had the same problem yesterday, pulled a "duh" moment on the water.  Got my boat up to speed and the engine was stalling could not figure out why finally it hit me ,  forgot to realese the vent screw LOL.

My boat has a vent on the starboard side, on top of the transom about 8" from the motor, looks like a little black cap. He probably closed it for winter. That oil filter is your water/fuel seperator.

I also want to mention that this is a permanent tank, not one with a venting gas cap.  

Some boats will have a cut off valve on the hose for the vent for storage. If it doesn't have that your vent could be clogged, or you are just trying to fill your tank too fast. I recommend making sure the gas nozzle is as far down as you can get it and if there's a curve in the filler tube that the nozzle follows that curve. If you're filling it too fast the fuel you put in is taking up more volume than the amount that can escape the tank through the vent.

Some boats will have a cut off valve on the hose for the vent for storage. If it doesn't have that your vent could be clogged, or you are just trying to fill your tank too fast. I recommend making sure the gas nozzle is as far down as you can get it and if there's a curve in the filler tube that the nozzle follows that curve. If you're filling it too fast the fuel you put in is taking up more volume than the amount that can escape the tank through the vent.

That did the trick.  I entered the gas nozzle upside down to follow the curve of the inlet.  I took a while to out in 12 gallons in the 16 gallon tank but its in there.

That did the trick.  I entered the gas nozzle upside down to follow the curve of the inlet.  I took a while to out in 12 gallons in the 16 gallon tank but its in there.

Glad to hear it worked for you!

I just bought a 1987 Fisher bass boat. I notice when I filled the internal gas tank that gas would just spill out on the overflow vent. It was so bad I couldnt put it in the lake.

That's probably just for that time, next time, at another pump, it will be blowing back on you again.  Sounds like you have one like mine, the vent goes back into the filler hose just below the opening or that re stricter plate so when you stick the nozzle in, the rubber splash guard restricts the venting, pull the nozzle out some to prevent that, and the gas stream is interfering with it.  

I've just gotten to where I poke it in, go full blast, let it spit back out a few times, dumping gas on the concrete, and it will finally make the auto cutoff in the nozzle quit tripping and fill the tank.

Might want to check the smaller hose going up, that's the vent, and make sure it's not sagging enough that gas can get in it and not drain back into the tank.  That will seal the vent off until it pushes that gas out and cause you to have the same problem.

All else fails, try filling from a 5 gallon gas container and big funnel and not a pressure pump.