A thin shmear of properly exposed UV cement around the perimeter of the xtal-to-case interface should do it. But, before allowing the watch anywhere near moisture again, I would have it pressure-tested (at least yearly); & I would also have the movement overhauled asap as the moisture you saw in the xtal may only be the tip of a slow wave of corrosion making its way through the movement underneath. A relative once of prevention now may avert a pound of pain later. At the very least, assuming the damage has not already been done, you might try placing the watch into an air-tight jar with some packets of desiccant for 24 hours, which may help to dry the innards out.