Flotation bags.

I’m pretty certain Eric Brown got out of at least one Martlet thanks to flotation bags. Pretty certain they deployed automatically via an immersion switch to avoid the pilot needing to do anything. 

What got me thinking was it a worthwhile use in fitting the bags, did pilots have time to deploy them and did they give pilots to get out of a ditched aircraft.

Douglas TBD-1 Devastator.

I have an inkling that the Fairey IIIF had them?

I suspect a lot of naval aircraft had them, certainly the Martlet/Wildcat did – were you asking about aircraft in general, RAF aircraft, or did you have particular types in mind?