Cap'n Tunabeard is a quest item that is needed for the Angler. This fish is obtained by fishing in the Ocean.
Yarr matey! Shiver me timbers! Avast, scallywags! There's a pirate captain who once had a pet fish named Cap'n Tunabeard, but during a big storm the fishbowl fell overboard! It has a hook for a tail, and an eyepatch and everything! You need to fetch me that fish, so I can be as cool as a pirate! Obviously it's out in the ocean somewhere! Duh!