No matter how pickled in varnish, wood on a boat goes thru considerable variantions in moisture content and moves a lot more than one would expect! A lot of guys use epoxy to glue, but I don't like it too well as it doesn't move with the part as well as could. My money's on the Poly glues, like Gorilla Glue, etc. . Nasty, stinky, foamy stuff. Actually, the foamy goop is what happens as it cures. Especially on an open-grained wood like mahogany, you don't want it getting on any parts you don't plan on...it will never come out of the grain. Lots of masking is in order! The incredibly strong bond that results has a bit of flexibility, so two parts expanding and contracting at different rates (whether due to material differences or a simple difference in mass between parts of same material) won't break down the joint.

In grain fillers, there are a couple of different ways to go---either the clear poly fillers, or a traditional laquer-based wood putty that can be thinned with LT to desired consistency. I like the Laquer-based if I have time to allow days for the solvents to gas off well (regardless what it says on the can). Spread on/work in with a bondo spreader, and wait until it is fairly stiff before burnishing off the excess with a piece of burlap...wait until it has had lots of time to harden and offgas fully, and then do your final sanding before varnish.

Dave