[Taqwus]

Independent summons ("Special Monsters") will cheerfully attack and slaughter any non-special-monsters that you teleport or summon into an enemy province. If memory serves, they may then disappear and not actually attack the province owners, but it's been a while. All Special Monsters will work together, IIRC, while same-side summons and teleports will work together.

Domes can be stacked, although IIRC the developers have stated that only one dome of each type is checked per incoming spell, so the stacking is just for insurance.

Versus underwater: check the spell manual. The usual cheapies Fires from Afar and Seeking Arrow will not work. Horrors, Ghost Riders, Earth elementals, Imprint Souls, however, will.