toastchee...I assume your CPAP machine is AC powered. I further assume you are looking to power it at anchor rather than at the dock right?
To use it onboard you'll need an inverter to make your DC batteries back into AC. First thing you need to do is find out how many AMPS your CPAP draws at 120V or how many watts...then we can help you with what size inverter you'll need AND whether your battery setup can handle it. What size/type amp hour battery did you get? If the CPAP uses straight AC then my guess is you can't do it off a single small battery...but if it plugs into the wall with a little "brick" DC adapter then you may be able to do it. More info needed!