Look up details about the Raytheon fishfinder first. Usually, the
transducer only needs a twisted pair with a shield. Chances are your
Ratheon unit also has some signals in there for temperature sensor or
speed. Chances are if you get the right signals connected then it will
work. This all assumes that the both units are 200khz and one is not
50khz. Some humminbirds were not twisted pair shielded but coax so
that could be a problem too although you could gnd one of the twisted
pair and have the humminbird drive it single ended with the coax cable.
If you're not concerned with sensistivity then it may work ok.

good luck.

Don