Could there be something in the lake that is making him stink? What does it smell like? If it was the ears, the odor would be just around his head and you would know. Same with anal glands, that's a very distinct odor. I do think you are bathing him too much, but I've never heard of that causing an odor. It can cause other issues like dry, flaky skin, but a bad odor seems odd. Try just rinsing him off with the hose after he swims or comes home from daycare and maybe do a full bath no more than once a month. Dogs don't sweat the way people do. They release body heat through the bottoms of their feet and by panting. That's what causes their feet to smell like Fritos. Have you checked his mouth? Sometimes it's their breath and it could mean he has a tooth issue.
My Luke had bad allergies and that caused him to always have a funky odor. I would bathe him and within 2-3 days, he would stink again. I think it was his body reacting to all of the allergens. I would bathe him once a month and give him cool water rinses at least once a week. But like I said, the odor never really went away for more than 2-3 days. Sometimes, I would use a doggie deodorizing spray on him in lieu of a bath.