I would forget about harness connectors to interconnect. That plug is proprietary to the stereo model you removed. That was not there when the boat left the factory. I would bet, that you can pull that connector you're holding in your hand. If you pull more out, I assume you will find the splices that connect that proprietary connector to the actual harness.
Most stereo's don't follow the same color coding, but they do have the same connections, in principal. It's a matter of matching the wires on the stereo with the wires on the harness. worse case scenario would be to cut the connector, but as stated, I would bet you'd end up with 2 splices a few inches apart on each wire.
It would be helpful to find the wiring diagram for the stereo and connector. If you know what 'blue' on the connector is, you can determine the corresponding color for that function on the harness itself.
Boating; The act of spending large amounts of time rushing around like crazy, in order to sit around doing nothing.