How about a pet mob that can catch fish and floating items for the player? They could catch floating items for a player busy on land or at the bottom of the ocean. Two variants could be added to spawn in oceans and freshwater rivers.
Wild otters would be shy like ocelots, but can be lured, fed, and tamed with clam meat (for sea otters) or salmon (for river otters).
To get the otter to fetch specific fish/items and to avoid getting spammed with items the player doesn't want gathered, the player could command the pet by giving (Q key) the otter an item in their inventory. The otter will pick up the item in its mouth and swim off to gather matching items. Any raw/cooked fish item the player gives the otter will tell it what kind of fish to attack.