locker construction

When it comes to cupboards and lockers I plan to fit a face frame onto the carcase. I will run a grove around the edge of the boarding and on the back of the face frame. glue the loose tongue into the back of the face frame. Assembly will involve pushing the face frame into position with glue and nailing through the carcassing with headless pins to secure it while it sets. This way I can spray the face frames before fitting.
Its how I make kitchens and works really well.

My question is what board material should I be using for the carcassing? veneered ply or mdf. Or can I only use a specialist board

 

There are several ways to build a boat's internal furniture. What you're describing seems to be "stick built", whereas you have a simple frame, usually of light fairly dainty stock and a skin is applied, which often is thin plywood, though PVC or other plastic, cored panels, GRP sheet goods, etc. can be used too. MDF has no place on a boat, it just breaks down. You'd be best advised to follow the plans for furniture assembly.

 

MDF on boats!!

Never, not ever ever use MDF ... I make my cupboards a similar way but my face frames are screwed (stainless screws) from the back thru a cleat on the carcase. I use structural ply (waterproof bond) 1/2" to 3/4" depending on where it is as my "carcase" doubles as hull framing, bonded to the hull with about 2000gm glass.
This way most of my interior is removable. Someday I or the next owner will appreciate it.