Sounds like the owner's problem, not the fire dept.
We don't have an SOG on this specifically because it just doesn't happen to the point of one being in place. If it was significant enough to garner a FD response, then it would be treated as a HAZMAT call. If the owner wants to try and dump it, well then they will be talking to the DNR.
The important thing is to handle this as a hazardous condition/fire prevention issue.
We have a few of these every year - usually someone carelessly mistaking a rod holder for the fuel filler spout. Some of the basics:
1) Don't let the owner - or anyone else - start the boat. Get the keys and get them ashore.
2) Don't let the owner flip any switches or activate any battery-powerd accessories.
3) Vent the bilge manually - don't use the bilge pump, as it can cause explosions or fires.
4)If the boat is in the water (most are) you'll need to double your RIT. One RIT should dress for firefighting and the other one should stage farther away dressed for water rescue.
5) If in doubt, use Class B foam to suppress vappors.
6) If a large amount of fuel is trapped in the double hull, there are a couple of things that can get it out. One is to get the boat out of the water (trailer or marina boat lift), get it away from ignition sources, and open everything that accesses the area where the fuel is trapped.
7) If there is a bilge drain, you can elevate the boat over a catch basin, steel drums, or something similar, pull the plug, and let the fuel drain out.
8) If one is available, you can use an intrinsically safe vacuum pump truck and a non-sparking stinger and vacuum most of the fuel out.
9) Most importantly, don't let the owner just dump the fuel, as John Crabbe says.
If that happens, it won't just be DNR involved, it will likely be the Coast Guard, too.
10) Ensure that all hazards are mitigated before terminating Command and releasing the boat/scene to the owner, harbormaster, or whoever is most appropriate.
Everything past 5) is generally the owner's problem, but the FD should maintain Command to ensure that any fuel offloading is done in a safe and environmentally sound manner.