Aquatic beasts of burden

Have been planning a Dark Sun style campaign, but recently we have done an adventure set in a desert so I want to add some variety.

The known world is a vast archipelago, tall volcanic islands are the only sanctuary from a vast brackish ocean. The land is dying, the sea is hypersaline - slack, windless, drained of love and life by the merciless Sorcerer Kings that plough the world of it's magic.

Small ships still ferry weary passengers between the attols, searching for the scarce amounts of drinkable water. The great warships are, of course, owned by the Oligarchs - powered by magical engines that can cross the flat water in a few hours.

The common folk such as you must charter tiny barges pulled by a pair of ...