> If you have a hose that you can fire from a fairly protected position and the guy with the RPG is completely exposed because he's trying to climb up the side of a tanker, yeah, I might.

In this scenario you are standing on a ship that is full of highly flammable oil.

There are more outcomes than just the hijackers gain control of the ship or they leave you alone.

If gaining control is no longer an option they could decide "fuck it" and just fire into the side of your hull.

Fire is always a risk, but crude oil is not "highly flammable". Some refined products and LNG are more problematic.

That might cause the oil to leak out. It's unlikely to start a fire unless the tank is fairly empty.