I just cut my aluminum tank out of my boat and didn't blow up. The tank was too large to pull out through the lazerette hatch.
I used a special hand saw ($10) I bought at Lowes that allows you to use sawzall blades. Used a 14 Tooth Per Inch sawzall blade. If I had to do this again, I would have used a longer blade (more efficient cutting, ie, less back and forth).
First, I had to cut away the fiberglass from the aluminum tabs that held the tank in.
Second, I had to make three long cuts (18 - 20") in order to cut a corner of the tank off (it was an odd shape) in order to get the tank down to a size that could be pulled through the hatch. It took an afternoon.
If you do fill the tank with water, remember that legally (I'm pretty sure) you have to dispose of the water as hazmat (like the gas). I would not want to risk my life using a sawzall, even after rinsing the tank out with water. I just don't know enough about how to make sure all the fumes are gone prior to using a heat and spark generating device on the tank.