Orca

It patrols hardmode ocean water and behaves like the Shark you already know how to deal with: straight lines, wide slow turns, and no way to follow you onto dry land. The counter has not changed. Stand on a platform or a raised block and it can do nothing about you. What has changed is the price of one careless swim, since it hits hard enough to end a trip back to the surface that a shark would only have made unpleasant.

Dropping a Chum Bucket into ocean water pushes their spawn rate up sharply, and this is the one case where adding more does nothing whatsoever. A single bucket already hits the ceiling, which is the opposite of how they work when you are stacking fishing power. Farming for Shark Fin or the banner takes one bucket and some patience, nothing else.

This is the only enemy the game has ever removed and then brought back, years and one console generation later. Real orcas are dolphins rather than sharks, and they leave swimmers alone.