• You can find random mini-islands when moving to another island or just traveling far around the map from the main islands.
• For easy Hobin Rood achievement, just use the Crab/Fishing Rod with no bait, then kill it using arrows.
• Not only Money in a Bottle, but Manta Ray and fish caught with Crab Carcass bait can be sold to Robin Hood. (I don't know if cook the fish first can increase the value)
• New fish and their drops can be found using Crab Carcass bait, which can be found on a mini-island with fish skeletons.

Leave any comments if you find something useful in v1.1.0

You can locate the mini-islands whether you are on an island or out at sea; listen closely and you will hear a beeping sound like the one from the markers on other islands even though nothing appears on the radar. Simply follow the sound to find them.

You can indeed cook "Euro" fish and sell them to any NPC, just as you can sell "Dollar" fish to Robin Hood.