Thermite is kid stuff. We used Willie Peter which is White Phosphorus. When they were hiding in Rice Paddies you fired some rockets in the area and they came up fast.....sometimes.
Dunno about the kid stuff.
I did a little reading up on WP. You can put it out with enough water to deprive it of oxygen. There is plenty of that at sea. It is more anti personnel than thermite and can leave nasty and painful burns, as long as it is supplied with air. This could be a PR problem with the touchy feelly crowd who would consider it cruel. I remember we had phosphorous flare ordinance aboard ship, and were trained to put them out with sea water and a snorkel nozzle.
Thermite generates it's own oxygen during the reaction. I was misinformed about the temps it reaches. It gets to 2500C (4500F). Thermite can be, and is used for welding metals, typically railroad track sections.
Water just makes it mad. It takes the oxygen out of H2O and liberates hydrogen gas. If there is free air around, you have a hot ignition source, Hydrogen gas, and oxygen, making a highly flammable event very likely. A ship's bilge often has water down there. Thar she blows...
Publicity about sinking a pirate ship would probably be favorable, as opposed to reports about phosphorus burns on people, whether they be pirates, or not. Har.
WP is controversial when used on personnel even in war time. But, WP makes such good smoke screens, few countries will make treaties disallowing its use, the US among them.
Anyway, it's hard to imagine kids playing with 4500F temps, and still becoming adults afterward.
Cheers,