Summon Minnows. Treat more like summon insects. They school in thick groups to suggest size. They would be very distracting and even blinding if "engulfed" effectively removing the target from play.
Jelly fish is a possibility.
Water elementals....wasn't there something in either the planes book or MM4 that used VERY small elementals? Those be used as a swarm I would guess.
If in shallow fresh water.... summon aquatic insects?