[Christina - how did your dogs take to hydrotherapy? Mine hasn't even swam yet, so I worried he will be really scared.
He'll take to it, well, like a lab to water!! Don't worry at all. There will be skilled help to see to your pup, and you will only have to watch and enjoy it with him.
One of my dogs went to a round hydrotherapy pool (after several strokes when her back legs were useless for walking), where she was supported by one of the staff while she swam round and round. At the end of her life the swimming and food were her only pleasures, so we took her for hydrotherapy every day. Before that, after she had had just one stroke, and seemed to be recovering, we used to go to a bigger pool that was also used for doggy fun sessions, and again the therapist supported her while she swam to retrieve her dummy. Recently hydrotherapy has become much more common than it was then, and at my nearest place there seems to be a whole team of therapists to see to the dogs. He will get every reassurance and a gentle introduction if he needs it, but the chances are he'll just swim without encouragement.
All my labs have been natural and very strong swimmers, and we have film of Vida where you can really see her webbed feet in operation!