If marine mode is selected but charts aren't available as active like Giomen suggested, check correct placement and naming is x:/Garmin/gmapsupp.img only for the datacard. Unless the datacard has been modified, maximum external memory available [hence even lower chart/map image size] is 512MiB for stock cards from Garmin.

A bit short on information so just guessing really. Initial post indicates the possibility that detail mapping charts aren't being read, and only the inbuilt marine basemap info is being displayed on the 276c. As the charts are shown in nuvi 1300 which can read 'any_name'.img in Map folder and also gmapsupp.img in Garmin folder, the possibility is that neither chart is correctly named and/or placed for use in the 276c's datacard as above.