If it's a 200Khz transducer it will likely work with Raymarine and some others that are 200Khz. Done this many times.
The center conductor of the coax cable is hot and the shield becomes ground. To confuse everyone the new Airmar transducers are often three wires, hot, ground and shield. When using an old coax transducer just make the center conductor hot and the shield the ground. On the ST-60 you can just ignore the shield input and use ground and hot and it will work fine if it is a 200Khz unit.
There were very few older 50Khz transducers unless for fish finders. Most of the old ones for depth only were 200 or 210 Khz. If it is a 210 it won't work well with a newer digital read out. I am using a 31 year old Radarsonics, 200Khz transducer with a 2010 Rayamarine ST-60.. I also have the Raymarine unit mounted as a shoot through for back up.
Cut the RCA or termination off the end and strip the jacket from the coax:
Use a mechanics pick to un-braid the coax shield wires:
Twist the shield nicely and turn it into it's own wire:
Wrap aluminum foil around the center conductor then place heat shrink over the wires and shrink. Then install crimp terminals to fit your display:
Plug the red/center conductor into the hot or "BLUE" if using an ST-60 and the shield or black into the ground or BLACK slot if using an ST-60.
BTW my 31 year old transducer was one of the few items to survive my lightning strike this summer even though every ST-60 device on-board was fried. The damn thing is still going....