Cracked Window

Unfortunately, that's a replacement. There's no way to repair broken glass aside of sealing it and maybe apply a piece of black aluminum vertically to hide the crack. That's a band aid though. Good luck.

Aside of being hit hard, the only thing I can think of is hull flexing in heavy weather or hitting a piling hard which could cause a flex. The window might have been installed too tightly into the recess and could crack with minimal flexing (which every boat does). They get installed with a flexible mastic so they don't break but if it were too tight in the recess, snap!

Discovered an interesting band aid or these things a ways back.
Boot & Trim tape. In your case, a single 1 inch wide black trim tape from West Marine.
Done well, will look like a small mullion or trim.
The tape last a long time.
Sticks and stays stuck for a long time.
Easy clean up.

Been there done that......more times than I can remember. Easier than the 1" wide black aluminum glued on.

Is the window a laminated safety glass? Is the crack also on the inside?

If the glass proves to be a laminated safety glass and the crack is only from the outside, you should be able to repair the glass with clear resin using a laminated glass repair kit if you want to do it yourself. The glass repair repair kit is sold by Liqui Moly. There are also contractors who offer the laminated glass repair service. I have done the repair a number of times on car windshields and the result are perfect. The cracks become almost invisible and are completely sealed.

You should carry out the repair as soon as possible before the crack collects dirt and air gets trapped between the laminated surfaces.

https://www.liqui-moly.com/en/service/workshop-solutions/laminated-glass-repair.html