Divide the class into two. One half creates music for the sea. This might be a series of slow-moving deep chords on instruments such as bass xylophones and keyboards. Experiment with making gentle wave shapes by moving the pitch of chords slowly up and down. The other half makes melodies for the sea horses on higher-tuned instruments, with faster movement (but not too fast) and gradual changes of direction.
Add other instruments - cellos, horns and so on - for the sea chords and experiment with chord progressions veering between major and minor - eg, Cmajor, D minor, E major, Fminor, G minor and back again reversing the tonality. Mimic the sea horse's colour changes with rapid shifts in timbre, eg from guitar to metallophone to clarinet, within one phrase.
Listen to Debussy's astonishing sequence of chords, the "call of the deep", at the end of the first movement of La Mer. Listen to the trombones slowly emerge from the broad instrumental texture. Can students create something inspired by this? For the sea horses' movements, experiment with trills to reflect rapid fin movements, and introduce repeated changes in time signature for shifts in direction.