Some islands are near the shore, or only cut off during high tide. Do ground sonar to locate the river of geothermal water, linking the island and shore.
We drill down to a convenient magma chamber, or drill down and site a plug of high explosive. Once exploded, we have created and artificial magma chamber.
Drill down to the magma chamber, and inject TiO fluid. The turbulence of geothermal water at high pressure, does Molecular Nuclear Fusion
So the water boils off, into steam. Which pushes the land up. Eureka – the land rises. Not more ferries – the island has a permanent land bridge.
My thanks as ever to Dr Zimmerman, and his thought about Nuclear Fusion: his uncle thought up sonoluminesence.