gpsmap 620 recognize only one map from SD ( gmapsupp.img ) no solution here for second map, only if garmin add support for second map.

You can split CN and add it in internal memory or you take two SD card, on one SD card add CN and on other Bluechart, so when you need auto mode you add SD with CN and when you need Boat mode, you add SD with bluechart, so simple is this