Green’s Dictionary of Slang

swimmer n.1

1. (UK Und.) a counterfeit coin [? the metal used was light enough to float].

[UK]B.E. Dict. Canting Crew n.p.: Swimmer, a Counterfeit (old) Coyn.
[UK]Grose Classical Dict. of the Vulgar Tongue.
[UK]Lex. Balatronicum.
[UK]Egan Grose’s Classical Dict. of the Vulgar Tongue.

2. a tender or guard-ship.

[UK]Morn. Post (London) 31 July 3/3: When Bill Soames heard that Sir Francis Slygo sneaked off from the Tower in a swimmer, he observed he was a shycock and that all his pals ought to turn him up.
[UK]Lex. Balatronicum n.p.: swimmer. A ship. I shall have a swimmer; a cant phrase used by thieves to signify that they will be sent on board the tender.
[UK]Vaux Vocab. of the Flash Lang. in McLachlan (1964) 273: swimmer A guard-ship in the river.
[UK]Egan Grose’s Classical Dict. of the Vulgar Tongue [as cit. 1812].
[UK]Metropolitan Mag. 14 330: He’s a poor crawling wretch, who works under the armpits, to be banded, and hour’d up in a swimmer all his best days, and then to be tatted.

3. a dumpling.

[UK]J. Keane On Blue Water 36: These ‘'water whelps,’ as we called them, are properly called ‘dough-boys,’ but our ‘grub-spoiler’ — pet name for ship’s cook — called them ‘swimmers,’ probably because they were such heavy sinkers.
[UK]Sunderland Dly Echo 24 June 4/6: Strolling into the Café Royal and ordering ‘a pint of “mahogany,” two “doorsteps” and a “swimmer”’.

4. one who retrieves drug packages which have been purposely dropped into the sea during importation.

[US](con. 1982–6) T. Williams Cocaine Kids (1990) 9: Import arrangements may involve ‘swimmers’ who retrieve packages from the ocean and ‘mules’ who transport (sometimes unknowingly) quantities into the country.

5. see swim v. (4)

In derivatives

swimmered (adj.)

of a thief, compulsorily enlisted in the Royal Navy as an alternative to serving a prison sentence.

[UK]Vaux Vocab. of the Flash Lang. in McLachlan (1964) 273: swimmer A guard-ship in the river. A thief who, in order to avoid prosecution, when before a magistrate, on condition of being sent on board the receiving ship to serve the king, is denominated by his palls, to have been swimmered.
[UK]Egan Grose’s Classical Dict. of the Vulgar Tongue [as cit. 1812].