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A lot of hot tubs use bromide instead of chlorine..
And I believe bromide is more reactive with gold.
There's a compound called gold bromide and if you read the description sounds an awful lot like what happened to your watch.
Gold(III) bromide is a dark-red to black crystalline solid.[1][2][3] It has the empirical formula AuBr3, but exists primarily as a dimer with the molecular formula Au2Br6 in which two gold atoms are bridged by two bromine atoms.[2][3][4] It is commonly referred to as gold(III) bromide, gold tribromide, and rarely but traditionally auric bromide, and sometimes as digold hexabromide. As is similar with the other gold halides, this compound is unique for being a coordination complex of a group 11 transition metal that is stable in an oxidation state of three whereas copper or silver complexes persist in oxidation states of one or two.[5]
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