This is a long shot, but it used to be common for folks to inadvertently select a geodetic datum other than WGS84. That would give you the symptoms you're seeing. I'm not sure on the 741, but on the 740 it can be selected by:
1. From the Home screen, select Configure > Units.
2. Complete an action:
• Select Position Format to set the position format in which a given location reading appears, and select a position format.
• Select Map Datum to set the coordinate system on which the map is structured, and select a coordinate system.
I've "fixed" this setting on a number of boats, but I think nowadays all chartplotters default to WGS84. I know the Garmin 740 does. Still, we all tinker with our electronics when we're first learning how to use them, and sometimes we change things without realizing it.
I've also seen us steaming across land on several different chartplotters. But only where I know the underlying cartography is the problem, such as on canals in the US and Canada. Where there are NOAA charts, this should be very rare.