It's hard to know what to suggest, not knowing the finish or even the wood or the age of your boat. If you have an existing finish that is starting to look lousy, you might need to start by either stripping or sanding. Then you might want to do some cleaning with oxalic acid or something similar if you have stained surfaces that don't come out by sanding or brass wool. I think that varnish would be a good choice for finishing if your interior wood is teak. I wouldn't use teak oil ... my experience is that it is too oily for interior surfaces.
It is a worthwhile project. I just finished varnishing all the interior woodwork in our boat. I removed all the trim and everything that was screwed in with exposed screw heads. I left the bulkheads in place as well as the cabinet trim that had screw holes filled with bungs because I didn't want the extra work of removing and replacing the bungs. Everything in my interior had been unfinished teak in extraordinarily clean condition after 20 years so I needed to do very little cleaning with brass wool and TSP. All the trim and the doors I took home over the winter for very light sanding to give it a baby smooth finish. I used a minimum of 4 coats of spar varnish and am very pleased with the results. Yesterday, I replaced all the trim and the boat looks fantastic.
