Hello,
ArcGIS for Maritime: Bathymetry is a tool to manage, visualize, manipulate and serve data coming from bathymetric sensors.
My understanding is that .adm files are a proprietary format from Garmin that doesn't contain bathymetric data. so my guess is that your best bet will probably be to contact Garmin to ask them how to export the content of these files to ArcGIS.
Thank you,
Bertrand