Not Cornwall and more aircraft than maritime, but there is the Fleet Air Arm Museum in nearby Yeovil Somerset.
http://www.fleetairarm.com/
A visit to Falmouth should let you see some RN ships in drydock or fitting out at the yards there. You can also visit the National Maritime Museum there
http://www.nmmc.co.uk/index.php?page=Visiting&id=9 and Pendennis Castle whis has some interesting WWII coast watch facilities amongst other interesting artillery exhibits.
http://www.cornwall-online.co.uk/eng.../pendennis.htm.
I visited Cornwall a couple of years ago with the family and had a great time there. A very beautiful and interesting place.
Also if you are interested in archaeology in any way make a visit to Carn Euny down near Sancreed on the Penwith Peninsular. I found that an amazing site.
http://en.wikipedia.org/wiki/Carn_Euny
EDIT: Was browsing the FAA site and found this:
http://www.fleetairarm.com/en-GB/exhibit/****e_Achgelis_FA330A-1_100545/6_30_59.aspx
