Return to Monkey Island

Doing this requires the Blowfish, which you can get by winning the burping contest in Act IV against the old Pirate Lords by drinking the green fizzy drink you get at the remains of the SCUMM Bar (if the drink isn't fizzy by the time you drink it, you need to get a new one).

After getting the Blowfish, go sailing and click on your ship at a random part of the map. You'll then need to make sure you puff up the blowfish and then drop anchor and jump into the water. There are 7-8 different ocean floor variants and they are random, and you need to see each one once.

So after going to the ocean floor, climb up the anchor, puff up the Blowfish again, and find another point on the map to dive back down on. Just keep going until the achievement unlocks.

DoG Suey ID pointed out that one of the unique variations is story related, so wait to grind this out until after you've obtained Bella Fisher's key.

deutschZuid pointed out that you don't need the Blowfish on Casual difficulty.