· Zephyr Cove is on FIRE!
Replace the bolt and run it. The other bolts should have kept it tight enough not to cause damage. I wouldn’t continue running it until you get a proper bolt though.
Water won’t get in the gear case unless the seal around the shaft is compromised. Water fills the inside cavity of the midsection anyway, no worries there. If you maintained water pressure and did not overheat the water tube stayed connected or at least tight enough to keep water to the powerhead. Get a replacement bolt, tighten the rest to torque spec and go fishing.