[Finalgenesis]

Merman swarm is really the cheapest and fastest way that early in the game, emphasis on the swarm.

barring rare exceptions and/or big bless, there isn't much in trinket on weak land commanders (or crap water commanders) that can solo a swarm of indies with UW penalties piled on top, you'd fatigue out and have problem hitting things.

A slight bit up the ladder at Const 2 will give you water breathing pills which your priest can forge, then just bringing say 2 pills worth of your elite troop of choice UW (ghoul guardians...etc) should work for UW expansion, if you have N, a single manual worth of troops is more then enough.