There are two sea creatures that have pouches.
The first is the seahorse.
Seahorse Facts | The Seahorse Trust via kwout
Male seahorses possess a brood pouch on their front side and when mating, the female deposits her eggs into his pouch. The male fertilizes them inside and carries the eggs in his pouch until they hatch.
Another sea creature with a pouch is the giant isopod (Bathynomus giganteus).
Giant Isopod - Deep Sea Creatures on Sea and Sky via kwout
They are crustaceans that can grow up to 16 inches long and inhabit the deep sea floor. Giant isopods reproduce by laying eggs and the females develop a pouch known as a marsupium, where the eggs are stored until the young are ready to emerge.
For more on sea creatures and marine life, check out the Ocean Animal Encyclopedia at oceana.org.