I am not an expert but I assume this is because it will reduce the glare reflected from the water.
e.g. basically you are looking indirectly at the sun via a mirror (the water).
Regular sunglasses are not made for looking directly at the sun but Polarized glasses should block these reflections from the water.
And as an additional benefit you will be able to look below the surface of the water for any obstacles or just to see how deep it is.
Regarding the breaking: if something is hitting your eyes hard enough it would break the glass and made it enter your eye you are probably in trouble anyway.
Also when water enters your eyeballs at high enough speed you can also get problems.
IMHO reasonably build sunglasses for watersports are way better protection even and far outweigh the risk you are mentioning.
I am personally using C-Line Davy Polarized Sunglasses and they are great.
They are comfy and the build in straps never made me lose one (and had plenty of crashes)