Hey 3rdman,
FWIW, I'd go with MikeG's 40+ years experience and use a foam core. My side decks are plenty stiff with the Corecell.
Agreed: properly mounted hardware etc. will keep the core dry, BUT, it's hard, nigh impossible to keep all water out 100% of the time. No telling where it'll come in, mounted hardware, port light frames, a hastily drilled fastener... It's nice knowing that no matter how badly I mount/seal my hardware, the deck core will NEVER rot again.
As to scarfing, my 2 cents. Cutting the skins first is the way to go. A lot of mine were still adhered to the core, and it would have been impossible to separate them without breaking had they been pre-scarfed. Also, a clean cut line gives you a nice reference for doing the nasty grinding for the scarf joints after the skins have been glued back on.
Doh! Capt Ebb's reply just came in. Hope I don't contradict his wisdom...
PS. Then there's Craig's post above - I'd like to have seen pics and have more detailed info on his POs recore/repair. All sorts of opinions out there...