Absolutely. Pump it into air intakes, flood the generators, do airdrops from helicopters and planes onto suspected entrance locations. Repeat until the ground is a soggy mess and sinkholes collapse the tunnels.
It would require a lot more water than could practically be air-dropped, even with the sea so nearby.
Exactly - seawater is cheap, it’s nearby, it’s relatively safe to handle for friendly troops, a containment failure isn’t going to be a real issue since the sea is right there, etc.