I suspect most folks don't get enough magnification if they use readers on the water.
I use 3.25 to 3.50 on the water...When on the water a short focal length is not a problem ...in a way helpful.
2.25 to 2.50 for average reading
Less than that when on the computer
And when I tie I use a head loop that has several magnifications.
It's just what works for me .