Not meaning to talk you out of this configuration, but before huge effort, consider this:
If you interface the AIS receiver directly to the Android tablet by USB Host-mode cable, that means that the battery
in your tablet may also be supplying at least some power to the AIS unit. Not good for battery
life. And if the AIS receiver has no other power source, battery life could be measured in minutes....
Just for your consideration.
Now, to the technical issue:
If the USB connection to the tablet does not emulate and interface exactly as some recognized serial port, then it will not be usable for OpenCPN
without creating a custom driver.
Test this by installing any of a number of serial port terminals available from the Playstore. If you install such a terminal app, and plug
in the AIS receiver, set an appropriate baud rate, and then see output on the terminal screen
, then OpenCPN will be able to use this device.
Please let us know how this goes.