AGE of the boat? Model? Any previous owner? History (was it a sunken or submerged boat/hull?)
Need more info, but ANY of those reasons can cause the glass on the cockpit deck to crack, from excess moisture under the deck trying to "escape". Do a search on my name and terms "vent bilge" for more info, but we sorely need MORE details and pictures from YOU ... to be of more help ...
If newer, see your dealer YESTERDAY ...