Green’s Dictionary of Slang

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

c.1698		B.E. Dict. Canting Crew n.p.: Swimmer, a Counterfeit (old) Coyn.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.

2. a tender or guard-ship.

1810		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.
1811		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.
1812		Vaux Vocab. of the Flash Lang. in McLachlan (1964) 273: swimmer A guard-ship in the river.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue [as cit. 1812].
1835		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.

1883		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.
1893		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.

1989

(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)