Water leaking around exhaust manifold bolt

[mhinnc]

The exhaust manifold was replaced last year just after we bought the boat due to corrosion on the older manifold which left a small hole near the aft part of the manifold.

Previous to our ownership someone replaced each stud & nut with bolts. When the bolts were removed for us by a mechanic either at that time or earlier the bolts tips snapped.

The first and second bolts (left to right) snapped far enough in the head that enough threads were left to tighten them well. The third didn't fair that well and when tightening felt "soft", but tight enough to hold.

After putting about 5 hours of operating time on the engine since, a leak has started, where a small bit of water bubbles around the bolt head (pictured here).
What would be the better way of dealing with this?