Assume the Hondex is connected to a depth sounder
and MaxSea can 'see' the depths? You will not be able to use MaxSea and OpenCPN
with the same connection.
If you shut down MaxSea and start OpenCPN the first step is to set up a connection with the Hondex with the same wiring
that was used by MaxSea. The user manual has a guide for making a connection.
Show the NMEA
debug window. If this window displays the data from the Hondex you can pause it and look for lines that start with something like $IIDBT or $IIDPT. As a first step the plugin 'Dashboard', part of the main OpenCPN program can display those depths.
Raster charts work
with OpenCPN but the coverage depends on where you are operating.
Let us know how the connection goes.