A shift in the magnetic field so that it's near impossible to navigate by compasses. In order for compasses to work, you'll need to enable compass needles to move and indicate direction in a near friction-less device. Perhaps an orb the size of a fist in which magic or new technology cancels gravity. You could even build on the historic application of mercury in light-houses to create such a friction-less device.
In the 18th and 19th centuries, the lens was usually set on wheels or bearings and attached to clockworks, which the keeper would periodically wind. In the 1890s, some keepers began floating their lenses in liquid mercury. The lens’s metal base spun more easily in the mercury, which helped the light rotate faster with less frequent winding.
And also:
Modern scholars have wondered if mercury, not isolation, was behind reports of lighthouse keepers behaving erratically or losing sanity, since chronic mercury poisoning causes confusion, depression, and hallucinations.
What could make a guild in your world special, could be the ability to withstand the negative effects of operating, not light-house, but compasses, that needed mercury or another material / magic/ technology to function.
The change in gravitational field could also be linked to the rising ocean levels in your world.