A loose flywheel nut, or striking an under water object, and that heavy flywheel just does not want to stop. as to an under water object, take your pick as to damage.
Most always a sheared flywheel key has a history of somebody has had it off and didn't reinstall it correctly. The flywheel and shaft tapers must be clean and dry. The nut must be torqued to 70-85 ft/lbs, using a torque wrench.
Lets hope it hasn't buggered up the keyways. You have problems if it did.
