Use the adjective peninsular to describe a near-island that is connected to the mainland.
Every good noun needs an associated adjective, and you won't have any difficulty in discerning where this one comes from. A peninsula is a piece of land that juts out into the water, nearly an island. Something that's peninsular looks like a peninsula or is a geographical area with a lot of peninsulas. The coast of Maine, for example, is peninsular, lined with thin necks of land. The Latin root, pæninsula, literally means "almost an island."