Move the cursor over the water, and left-drag in smallish circles. You’ll see land begin to appear; click repeatedly in one spot to increase its height. Middle mouse clicks will flatten land into smooth, consistent surfaces - for building to be possible; you’ll need some of that. Right click to lower the land and create a sort of ramp up onto the island, otherwise land units will struggle to reach it. You’ll notice the result looks like a boobie. This is hilarious.