Underwater Province Defense?

[Panpiper]

Recruit local, non magical leaders in the underwater provinces. Give them a little bit of local chaff for support. Equip each of those leaders with Rod of the Phoenix, Spirit Helmet, Boots of Quickness, and whatever else you feel useful to keep the hero alive and functioning. Give them the battle orders; 'Cast Spells'. You won't need many to provide a very effective deterrent against invasion.