the simplest way i've found is to fish a hose thru the hull drain plug opening* and into the top of the tank, just remove one of the fittings**, and siphon*** it out. you cant usually go up and over the gunnel, apparently physics wont let you.****
*first remove boat from water
***do not inhale gasoline or the fumes
****do not do anything dangerous or harmful to yourself, others, equipment, or the environment or anything that would cause me to be liable for any advice given. Always wear proper safety/protective gear, and dont powerturn. this has been a public service announcement.