The boat's in the water so I haven't taken the seacock apart to look at the rubber and have no idea what it looks like. I just know that the one feeding the generator doesn't work at all. (turning the lever doesn't do anything the water just flows out full speed)

Do you believe it's possible to fabricate a new piece If I can get the material?

If it is I'm ready to jump in the water, put a plug in the hole and take it apart.