Forest is a render, not a shoreline prop
Walkthroughs agree: you follow coordinates, you do not scan the horizon for a second landmass from the dock. Patch 1.0.5 removed sailing to islands you have not unlocked, so a missing mesh is usually a missing quest step, not a skip.
After Forest is open
The next gate is 3/3 ground leeches, then Modified Leech → Giant Piranha → skeleton hand-in. Killing the piranha without the skeleton talk does not place Island 3 on the Radar.