I agree with everything JRob said.
to answer your question, all hair replacement glues are water RESISTANT, some like ultra hold are water proof.
yes, after a shower (especially a hot steamy one), the bond will weaken and soften , it hardens back up once completely dry.
It would be helpful if you could post some pics, if you have receding hairline and a good supply of hair everywhere else, you perhaps may want to go with a frontal partial.
DebbieS.