WordNet sense 1 (beneath the surface of the water):

underwater, submersed

WordNet sense 2 (beneath the surface):

subsurface

WordNet sense 3 (growing or remaining under water):

subaqueous, subaquatic

WordNet sense 4 (operating or living or growing in water):

aquatic

WordNet sense 5 (under water; e.g. at the bottom of a body of water):

sunken