After differents tests, the results : I washed it with soap, alcool, etc.... and always the same : allergy came back.
But during my holidays, i have tried to paint the back of bracelet with clear nail polish (special polish for this use bought in pharmacy) : i have weared it 2 weeks long on the beach, sand, water, 25m diving, and everything it's ok now (i applicated a new polish every 3 days to be sure to have good protection).
Thanks all, cause my sub is now on my top3 prefered watches