Hi Adam,
I am in the same situation and will fit an Airmar CS4500.
It does work with the B&G Hydra 2000.
There are three sets of connections on the Hydra processor for boat speed :
Paddle wheel, Sonic, Ultrasonic.
The CS4500 connects to Ultrasonic, whereas the sensor you have now is connected to Sonic.
The CS4500 comes with an interface box that connects the sensor to the Hydra 2000.
You cannot use the P79 in hull sensor with the B&G Hydra 2000. The frequencies don't match.
If you want to use the P79, you need another display such as Garmin or Raymarine.
I am going to make another hole in the hull, just offset from the depth transducer, for the CS4500 when we haul for antifoul in November.
Cheers
Alan
Elyse SM437