What kind of keepsake can be brought back from the ocean?

There are many things that can be found on the shore. You will have no trouble thinking of them.

Pearls grow in oysters or other bivalve mollusks under the water; it is not likely you would find one on the shore, after the animal has died and the shell has opened. But unlikely does not mean impossible.

I think there is not a lot known of species that inhabit the open ocean and do not live near shore.

One thing I can think of is the albatross, a very large bird that spends most of its life over the open sea, only returning to the island of its birth to breed. Albatrosses often do follow ships, though, so that it would not be surprising to see one out at sea. And since birds do moult their feathers, it is conceivable that one might fall onto the ship, and you could pick it up. The flight feathers would be too large to carry in a pocket. But you might get one of the little feathers that cover the body, or else the tip of a feather that was broken off; this can happen too. You certainly would not find an albatross feather on land unless you went to one of the remote locations where they breed.

The first time I went to the ocean with my husband I found two shells that were similar and I made necklaces out of them :) there were lots of beautiful shells that would easily fit in a pocket

Sand dollar

sand dollar

a shell, rock, coral