How to depict atypical magnetic fields on a map

[ManOfSteel]

Unless...
there are a series of submerged islands in the sea, perhaps an entire mid-ocean ridge with massive iron deposits within granite domes. Not only are the currents between these barely submerged islands strong and treacherous, but when there are storms...and confluences of warm and cold currents in both the northern and southern hemispheres, as well as the tropical updrafts ensure that there are often storms... the sky is obscured and thus navigation by stars is impossible. Even worse, these terrible storms form within a matter of minutes and huge amounts of lightning are drawn to the underwater metallic deposits. The lighting generates sporadic magnetic fields which can make a compass useless for hours or even days.
I could imagine some ships encountering days of bad weather, and not seeing the stars for several nights, all the while battling storms at sea while trying to avoid the shoals. When the weather finally clears, they could be miles off course...if they haven't hit a rock or been struck by lightning. Lightning is attracted to the tallest object around you know, and those ships' masts are pretty high. Surviving a lightning storm wouldn't matter much if your sail was struck and burned. A ship off course, without a sail is not a good place to be. All you need do is examine one of the skeleton inhabited derelicts lodged on the rocks or read their logs describing their cowering below decks through night after night of lightning, fearing electrocution or fire, and their slow death from dehydration.
I'd be scared to venture into that area.