Lining-out a sailaway
There are no hard-and-fast rules, so it depends upon the depth of your pockets and whether you intend to hang anything on the lining.
Thin ply is probably okay for the roof as long as the fixing centres are perhaps 450mm or less apart. The curve will help it keep its shape. I suspect you will want/ need to fix things to the cabin side above the
AI have just bought a sailaway, what thickness ply should I use for lining it out? gunwale, so 9 or 12mm there will give you some thickness for screws.
Below the gunwales, use the same if you intend to fix radiators to it, if not you could get away with 6mm or less. If you know where any attachments will be, you could glue a reinforcing piece on the back of the ply.
Also, have you considered the thermal insulation, the need for a vapour barrier behind the ply and the need to ensure the ply never comes into direct contact with any metal part of the boat structure? If you use oak, remember that iron/steel contact will produce black stains so use brass screws.