When you find your boss, Kyth Mezereon, she'll task you with going to the island and stopping Terlissa Feeg. By way of helping you in this quest, she'll entrust you her amethyst eyestone.
How do I get to the island?
You can take the boat from the marina. But you need to get the key from the woman sleeping there.
It will take magical stealth to take the key without waking her up.
Only the amethyst eyestone will help you here.
You can use the owl amulet to move silently, or use the spider amulet to become invisible. Either way, the marina lady won't notice you. Once you have the key, go out to the dock, get in the boat, put the key in the ignition, turn on the engine, and pilot the boat north over the lake.
How do I navigate the island?
Follow the path into the woods.
In the woods, look carefully at the slime to see what direction it's coming from.
Go northwest and you'll find a portal.