Not (yet!) encountered that problem. Depends on how well you want to seal the watch. You can buy watch "o" rings from somewhere like
http://www.cousinsuk.com/catalog/4/2/1897.aspx a fiddle around with them, cut and superglue if necessary. But any sealing like that is pretty hit and miss and I wouldn't risk taking it outside on a damp day.
There are silicone instant gasket products for car sealants - see your local auto factor. Bearing in mind that A: this is hypothetical and B: I have never done it nor do I intend to - here is how to do it!
Place a slab of plasticine or similar in a can - a Casio presentation tin is about the right size. Remove module from watch case. Make a mould of the O ring groove by pressing watch case onto the flat plasticine surface. This will give you a negative mould.
Use a fine plaster of paris or similar to cast a positive mould of the watch case groove. Once totally dry and hardened use the instant gasket with the nozzle cut as fine as possible to pipe a line of gasket all the way around the o ring groove of the mould. When fully dry and hardened remove the o ring, trim if necessary
Re-assemble your watch.
Do you reckon that would work, Terry?
PS - get a cheap beater to practice on!