Warships became fortifications mainly through necessity

How often were ships used as fortresses in place of land fortifications?

Rarely, and usually as an expedient when a beached or grounded vessel offered the best available fixed position.

Navies did not normally build mobile ships merely to substitute for land forts. The practice appeared when a vessel had already beached or grounded and its crew could turn the hull, weapons, and protection into a fixed strongpoint. Modern landing craft placed on disputed South China Sea reefs offer a related example of using the platform that can physically reach and hold the site. The common feature is expediency: a ship becomes a fort because local geography and necessity make it useful ashore.