[katieorielly]

Eventually I discover from Raymarine EITHER you input s-video OR you input composite video! I need both, one s-video and two composite video. My Raymarine input cable has four individual composite leads, surely there is a way to join two of these into the s-video source?

[AV_Pro]

The reference manual says that the video inputs are set like:

1 Composite
2 Composite
3 Composite
4 Composite

or

1 Svideo 1
2 Svideo 1
3 Svideo 2
4 Svideo 2

Since the Raymarine Svideo cable only has two svideo connectors on it, you should stay with the compsite cable and get an svideo break out cable. This cable takes the little 4 pin connector on your source, and splits it into two female BNC connectors, for Y and C aka luminance and chrominance.
So you would then connect your setup like

1 Composite
2 Composite
3 Svideo Y
4 Svideo C

You have to set up your video application window accordingly. Its in the reference manual off their website.