Thanks for the tips EBarn. I went ahead and shaped the bottom with the mill yesterday--which went pretty well. The seam starts 3 feet from the stern, where it's pretty shallow, and runs all the way up the boat log and keeps on going, , and at the bow end it's pretty deep (judging from the next log up). It's also not really centered, but runs within 2" of the centerline. Thanks for the tip about how to put a stop in, I will do that where it stops near the stern. At the bow I guess I'll tie it together. And I guess I will have to put some kinds of floor timbers (courtesy title, they'll be pretty light) to hold it together, and use your crack-filling formula (when the warm weather comes.
I flipped it over and today I'm going to mill the sheer the same way I did the bottom. It's a nice revelation how handy an Alaskan mill is for cutting a curved surface within 1/8" of level. (To provide a level working surface, I levelled off the tree's stump and squared the end of the "boat"--very convenient and stable).
EBarn, you suggested: "If it is fairly dry, then toss it in the crick for a while [a couple of weeks] and see how much it swells up". Um, I could do this, it's sitting right on the creek bank, but why would I want to, exactly? Do you mean that the seam might close up? It might, (it's only a fat 16th wide at most), but it will still be there...and it ain't going to glue itself back together...eh?
Cedar is of course highly rot-resistant, but only the heartwood, not the sapwood, which I guess jives with your explanation--the sapwood layer is not very thick.
It's already starting to look like a boat, kind of. A real skinny one. OK, a badly bent rough-shaped torpedo...but that'll work!
SRS, sinz you esked, I am in the Ottawa Valley of eastern Ontario. Outrigger dugouts are of course the traditional native watercraft of this area, for enjoying the ocean surf, whaling, and repulsing the white devils...(not). I plan to take it down certain stretches of the Mississippi next summer. (Why not, it's only a mile away). Have I got you confused yet? For a musical reference to this Mississippi, look up "Whalen's Fate" or "Lost Jimmy Whalen", both 19th century ballads about the sad fate of a river driver twenty miles upstream from here.