You can get a free version that allows one map section to be downloaded at a time, and a version that has unlimited map downloading for your region.
I did that for a while and moved up to Master for 10 bucks a year. It has all the NOAA maps updated quarterly and also has all the USACOE depth survey data superimposed on the screen - as well as the wifi connection to the vesper unit.
Check out the Panbo review at https://www.panbo.com/aqua-map-maste...oute-explorer/
It comes with a GPS antenna, so I hooked up my old radio to the AIS unit, so my radio DCS was functional, using the MNEA0183 connection.
Lots of skinny water between my summer and winter spots, this USACOE survey function has been really helpful