Merchant of the Skies

To the west of your starting island you’ll find an island which is actually a giant talking fish. He’ll talk to you about apples and water, but really he just needs water. You’ll have to add a Buckets upgrade to your ship to collect the water (you cannot mine it like other resources) and go to a Water island (western edge of the map on my game). Collect the water and go back to feed Sam. You can only collect 190 water at a time and, annoyingly, he needs 800 to be fed. He’ll disappear for a day or so after you feed him so just snag water when you can and drop it off when you’re nearby.

When you hit 800 the achievement will pop, Sam will reward you and start swimming slowly around the map. You can keep feeding him if you want, but it didn’t seem to do anything. He talks more about apples but I couldn’t find any way to give him any.