Green’s Dictionary of Slang

deep-sea diver n.²

a £5 note.

a.1945		L. Payne private coll. n.p.: Fiver Deep Sea Diver.
1983		R. Aven-Bray Ridgey-Didge Oz Jack Lang 11: The freight to Balmain had not left him with much shrapnel out of a deep-sea-diver.
2000		market trader East London 4 June [personal communication] Who wants these sirloin steaks. There you are. A fiver, a bluey, a deep-sea diver.
2002		K. Lucas ‘All my life I’ve wanted to be a Barrow Boy’, in Obfuscation News Apr. Issue 20 🌐 You don’t get much for a Deep-Sea Diver these days.
2013		T. Peacock More You Bet 66: A ‘$5 note’, or ‘five dollars’, or a ‘fiver’, in rhyming slang was, and is, a ‘deep-sea diver’, and latterly, a ‘Stuart Diver’.