My Sam has swam a lot, salt water and fresh and his fur is reddish, too. I do not think it is from the sun, or the water. It pretty much grew in that way. He was partly shaved when I got him. I would have to trim off a lot to lose all the red! : )
I think for some dogs it is sun, and for some it is just their color. I met a beautiful blue-black newf once. I have had Sam 5 years and he has never been that color. Interestingly, she said her dog has never had the reddish tint. I have a friend whose hair is the exact same color as Sam's and she hates it. Says it is brassy. Has tried to die out the reddish tint for years, mostly unsuccessfully. But I have also known dogs and horses whose hair turned red from the sun and would stay their true color if protected from it.
I am pretty sure that the newf standard allows for some red in their coat...
I love the blue black coat and bought some black on black, but I never got around to actually using it. : D So Sam is still reddish. And I never rinse out the salt water, either. : )
Marie