We just used a standard group 24 deep cycle marine battery on our C-18. Plenty of power to run the auto pilot, VHF and GPS on a 7 hour crossing to Catalina Island. The group-24 is a tight fit, but it's secure. I wanted to remove the factory battery box to allow either a group 27, or maybe a dual battery set-up, just never got around to that project. But once I installed a charger on my outboard, never worried about battery power again.
Don