The Global Positioning System (GPS) is used worldwide for marine navigation in support of hydrographic surveying operations, where horizontal positioning requirements are typically better than 10 m (95%). Differential techniques are used to reduce errors associated with signal propagation through the dispersive ionosphere and to achieve such accuracies. Two such methods are differential GPS (DGPS), in which range corrections are derived for a nearby reference station and are applied at the remote user location, and wide-area differential GPS (WADGPS), in which ionosphere range errors are modeled over a large area using a sparse network of GPS ground stations. Both DGPS and WADGPS (Wide Area Augmentation Service) positioning accuracies are investigated here, throughout North America, for a severe geomagnetic storm event in 2003. DGPS horizontal positioning errors of 10--15 m (95%) are observed during this event, and WADGPS positioning errors generally exceed those for DGPS. This is attributed to the sparse WADGPS reference network and associated limitations in resolving the severe ionosphere gradients.