[Rich]

I'd be sure to sniff (with your nose) the bilge and be sure you are not smelling any gasoline fumes. If no fumes, then it's safe.

Next, I would shut off the battery switch (if you have it) before connecting the jumper cables.

Be sure that every load on the boat is turned off, this is all lights, radios, electronics; basically every electrical load should bed turned off.

Connect the dead battery to the jumper cables from the good battery. Be sure the good battery's engine is running now to charge the 'bad' battery.

Then let the charging system run for a short time while connected to the bad battery.

Next turn on the battery switch, this should keep electronics from seeing a surge or noise from a 'spark' type of connection.

Then attempt to start the engine. If it runs, let it run for a bit.

If it won't crank, wait a while and try again. If it still won't crank, you need more help than a quick jump.

Be careful removing the cables, don't allow them to short to each other or to anything on either boat.

Basically, this is the same you would do on a car, but you need to be extra vigilant to be sure there are no gasoline fumes around. Remember gas fumes will be at the bottom of the bilge as they are heavier than air.