The following isles cannot be accessed normally by a boat.
Coconut Isle - Bring 100 coconuts to the NPC on the western beach of Twin Forest Isle.
Dungeon (Prison) - Talk to MrKeel in the Dumbbell Dockhouse and pay $500. To get out of the Dungeon, solve 4 easy math problems. You will be released at a random isle.
Flurry Isle - Talk to the penguin on the south part of the beach of the bottom center wedge on Crystal Isle and pay it 1k.
Gas Isle - Click on the closest venus fly trap near the Swampy Isle Dockhouse.
Hidden Isle - Talk to SableFur on Crater Mountain Isle and pay 10k.
Islands Lake Islands - Feed SwimmerTurtle on the east bank of the lake and feed him strawberries.
Lost Isle - Click the Shovel on Decay Isle near the mausoleums.
Narner Isle - This isle is not viewable on the map. You gain access to the isle during ProfLongStorySon's Package quest by clicking on the wardrobe in ProfEuclidean's house on the first floor (AngleTon). Wild horses cannot be found on this isle.
Oak Isle - Go to the cave at the mouth of the river on Deep Jungle Isle. Complete the lever code (3,2,1,2,3,3,2). (You will need a key from MissLime in NarrowTon Inn.)
Timbuktoo Isle - This isle is not viewable on the map. You have a random chance of accessing it through boat trips from any dock or rowboat.