what is the best way to drain water from a marine gas tank(inboard)?

If boat is on trailer raise the front of it as high as you can get it. Pull off the fuel line at engine and attach an electric fuel pump to it and start pumping out the gas. Every so often fill a clear bottle .If it looks cloudy keep pumpning till it clear or if you tip it to the side slightly you will see the water at the bottom of the bottle. Keep pumping till you pull straight gas out.

Then change your fuel/water separator and add water zorb or either Starron additve to it . Carry a spare fuel water separator with you and try taking boat out for a ride .

You should be ok

If the boat is in the water you can do the same it might just take more pumping to get it out since water is heavier than gas and settles to bottom .Make sure you use good drum or container and dispose of it properly

The only way to get the water out is to pull the sending unit out of the tank .lift boat up in front.Put the hose with primer bulb no electric or the water won't be your problem. put the hose in the rear of tank stars pumping run till no water remains.Hose must be long enough to reach out of the boat an lower than the bottom of the boat or it will not gravity feed. If you try removing threw the pickup there will be 1" of water left in the tank. When threw add a bottle of dry gas to get the few drops left. Change the fuel filter & clean the 1 on the eng.

If you can see the water in the bottom of the tank get a friend and a wet/dry vac. Take the sending unit out of the tank, then put your hose in the hole all the way to the bottom. Have a flash light so you can see the progress. Have your friend turn on the vac and watch the water be sucked right out. You will have to be quick at the switch with this method but it works. Been there, Done that. Good luck

may be a defective drain valve or in easy terms some obstruction interior the valve. recommend turning the gasoline valve to pilot and permit the water to relax fairly, so which you will no longer get burned. connect an prolonged hose onto the valve putting the exhaust end on the exterior of the domicile. turn the valve on totally and back off. verify to be sure if it now shuts off thoroughly. If no longer after a pair situations of this, you are able to replace the valve. If the home is newly built, touch your contractor or the vendor.

Syphon into 5 gallon cans with a syphon hose. Then remove the fill hose & put an absorbent cotton towel in the tank to absorb the remaining water. Remember to hold on to one corner. Wring it out & repeat the procedure if necessary.

Water is heavier than gas, oil or diesel.