Nisha has never been swimming inside, but she loves the lake! Have you ever tried a baby-pool or played in the hose with Gary? Does he ever try and get in the shower/bath with you; if so, these may be good signs that he likes water
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I think he will do great; keep it positive and don't let him freak out. When introducing Nisha to water, everything was done on her own pace. Of course they cant wait forever with Gary, but maybe bring his favorite treats along to help "coax" him in. Please do let us know how his first time swimming goes! They make doggie life-vests; he may feel more comfortable if he floats.
***Good luck Gary!***
Did you ever take him to get x-rays?