The achievement requires that you "Hit each type of underwater mob with the fishing rod."

These are:

• Drowned vine man
• Drowned skeleton
• Drowned zombie
• Drowned Necromancer (WHICH YOU CAN'T HIT)
• Drowned slime
• Pufferfish
• Turtle
• Squid
• Glowing squid (found at the end of secret level Raviant ravine )

The achievement is bugged because since the last update you are unable to "hit" the necromancer. Your rod will ignore it completely or will just go through it.