Underseas rhymes with:
overseas, apc's, c's, cees, foresees, francies, jaycees, lessees, licensees, overseas, pc's, pcs, powerpcs, powerpcs', saez, scorsese, sea's, seas, sease, sees, seese, seize, sies, tennessee's
Underseas sounds like:
under-age, underage, undercook, undergo, undergoes, undersea, underuse, undress