Why pressurized-air drag reduction remains difficult for warships

Have navies tried injecting pressurized air under a hull to reduce water resistance?

Yes, but neither air-lubrication nor hovercraft-like variants have yet proved practical for normal warship operations.

Hovercraft-like approaches and attempts to use pressurized air can both reduce hull resistance. The physical idea is credible: change the boundary between hull and water so the ship experiences less drag. The challenge is maintaining the effect reliably while meeting the manoeuvring, endurance and damage demands of an operational vessel. The technology has been tested, but has not yet proved practical for ordinary warships.