Thank you very much everyone for your help, I'll remove the red (+ve) cable from the 10A ESC as this seems to be the simplest option given both ESCs have a built in BEC.
@RNinMunich I currently have 2, Bizmork and a smaller but similarly themed one named Gork's Revenge. You can find photos of both on the linked Facebook page. I plan to upload photos of Gork's Revenge here when I find the time to modify/rebuild her.
It was my first 'proper' attempt at building a model boat and I've learned a great deal since building her and want to go back and redo the internals including as big a motor as I can squeeze in for towing.