Going from memory....
The S1 is a Raymarine autopilot (I have the ST4000) and Seatalk is a proprietary Raymarine interface, so it only works with other Raymarine equipment.However, you CAN interface the two units using NMEA 0813, which will send cross-track data from the GPS/plotter to the autopilot.You don't need anything special as far as cable is concerned - especially stay away from anyting that is branded, packaged and labeled by one of the manufacturers. Just plain marine grade signal wire (I THINK it is only a single pair, but your owner's manual will let you know for sure) is perfectly adequate. I also doubt that the data rate is anywhere near where you would want/need shielded cable.I made that connection with my Garmin GPS, and it works just fine. Watching the GPS and autopilot work together (rather than fumbling with charts, sightings and steering) is just plain FUN for this techie.

Cheers,Bobs/y X SAIL R 8