I've looked in my pharmacy books and haven't seen anything related with water loss in muscular tissue.
This may have occured more with 1st generation of antihistaminic, those that causes sedation.
But, if you look at Claritin or Allegra, which are 2nd generation, it shouldn't be much of a trouble.
The only adverse effects which might be linked with water loss that were reported were dry mouth (dryness of mucuous membranes) and feeling of thirst. If you always keep well hydrated, it shouldn't be of a concern at all.
Good workout bro !