Solid piece! Functions well as underscore and has some neat layering in it. Digging the delayed introduction of percussion and the simple rhythm line to keep things moving along. Overall choice of instrumentation is good, a lot of different melody and supporting lines spread across a variety of instruments. Fits with the theme well - some of the voicings here do a great job selling the underwater vibe, particularly the mallets I think.
Good job, I think this is solid as a whole. Best of luck!