Water resistance

A watch keeps out water and dust where its case opens: at the back, the glass and above all the winding stem; the 1925 Perregaux-Perret patent names the joints around crown and stem as the easy path for dust and damp, especially in small watches.^2, 3

Screwed cases are older than the Oyster. Watkins lists Aaron Dennison's English patent no. 356 of 1872 for a case with screwed bezel and back, and a screw-down crown patented by his grandson in 1915.¹

Wikipedia says a waterproof case by Charles Depollier was patented eight years before the Oyster, and that Tavannes made a 'Submarine' watch sealed by other means.⁷

Another route was to have no crown at all: John Harwood's self-winding watch of 1923 had a case with no openings, its hands set by the bezel, so that it was 'practically dust and waterproof'.^8, 9

Paul Perregaux and Georges Perret applied on 30 October 1925 for a Swiss patent on a screw-down crown; its British version was granted to Hans Wilsdorf as their assignee.^2, 7, 10

Wilsdorf added his own patents in 1926: a crown that slides on the stem and screws onto the case, and a case whose back and bezel screw onto an inner ring, 'practically watertight' even in a shaped case. Together they made the Rolex Oyster.^{3, 11, 12}

Rolex calls the Oyster of 1926 'the first waterproof and dustproof wristwatch'; Watkins and Wikipedia point to the earlier screwed cases of Dennison and Depollier.^1, 4, 7

Testing followed sealing: a Swiss patent filed by Wilsdorf in November 1934 covers an apparatus for testing the tightness of watch cases.¹³

The swimmer Mercedes Gleitze became Rolex's proof. Rolex says she wore an Oyster across the Channel in 1927 on a swim of over ten hours; Wikipedia says she wore it on the failed 'vindication swim' of 21 October 1927, after doubts about her successful crossing of 7 October.^4, 7, 14

On 24 November 1927 Rolex launched the Oyster in Britain with the front page of the Daily Mail, and on 7 December Wilsdorf warned imitators in the Swiss trade journal that he owned the patents for the screw-down crown and hermetic cases and the trademarks Oyster, Aqua and Submarine.^7, 16

Omega's Marine of 1932 used a double case sealed with cork; Omega says it was tested in Lake Geneva at 73 metres and later found waterproof to 135 metres, and calls it the first commercially available divers' watch.^17, 18

Standards came much later. ISO 2281, in its 1990 edition, covered watches for 'ordinary daily use' including short spells under water and excluded divers' watches; ISO 22810:2010 replaced it. Divers' watches have their own standard, ISO 6425.^{5, 19, 20}

A 'water resistant' mark records a static pressure test on samples, not a promise for long use at that depth; the ISO standard discourages the word 'waterproof', and Wikipedia says the US Federal Trade Commission has barred it in advertising since 1968.^21, 22