Hello, does anybody know how to view an OSM "open sea map" already converted to .img file in garmin GPSmap 722, with software version 14.30 (newest in feb 2019) ?

This map comes from an open source of information, so it shouldn´t be banned to load it and view it in the device i have handy.

The map version by default in the device is "worldwide shaded relief map v2019.0 v7.00" (last version upgraded with "Garmin Express" through SD card).

When i try to read de .img file downloaded from [Only registered and activated users can see links. ], the device throws "cannot authenticate map" , and it doesn't load it or show it.

Does there exist any way of loading an open sea map, which is free, on these latest versions of garmin devices?

Maybe through the system folders and files of "Garmin Express" installed on the PC.

The files that come along with a map downloaded with "GE" are, as an example

mapname.gma (3 KB)
mapname.img (680000 KB)
mapname.unl (1 KB)

Opening with notepad, the *.unl file contains the string RC8XZ44BKZLGWX8FBYQEBZLGS, maybe this is a serial number for unlocking the *.img file. Could there exist a way that i generate the .img file along with these other .gma and .unl files ?

Thanks in advance to all who can help.