Having lost some decent sunglasses swimming in Aruba, and another set kayaking in Hawaii, I could probably use an insurance policy.
Sounder Goods is the original leather sunglass strap, Made in America and available in several tones, thicknesses, and lengths. I prefer the standard length Montauk in dark brown, since it doesn’t hang too low, isn’t interrupted by a knot, and feels classic, but you’ll have a lot of options to pick from, even a Bolo.
If a strap is out of the question for you, you could also check out sunglass brands like Sunski and Knockaround, which are priced to lose, or WavesGear, which float.