Some planes have an emergency mode for this, activated by a "ditch" button. It closes several valves which are usually open all the time. This way the plane stays afloat a little longer to give more time for evacuation. However as you may assume that is a button activated once as even if it's close enough to a shore the plane usually is way too heavily damaged to return to service ever again.

Other than that: Flying boats, comparatively common in the 20s and 30s aren't made anymore. As more and more airfields were built and every new generation of planes had increasing range and reliability they lost their advantage, while their disadvantage (being a bad ship and a bad plane) remained and couldn't be changed.