William R. Watt wrote:

ever since I started capsizing racing dingys in the late 70's I've been
using pieces of string with a loop in each end over my eyeglasses when
boating. I've been in the water many times but never lost a pair of glasses.
I'll admit I've never rolled a kayak in fast moving water.

I always use some form of retainer, but I've had glasses come off
nonetheless. I've found that even with a floating strap, they can be
lost easily.