Jeff--
If you're cruising marshes the flat bottom is great for minimum draft and flatwater stability. I'm sure it also helps in simplifying building.

The Chewonki design, at almost 20 feet long, would seem awkward to maneuver in tight spaces. Unless you're a really big guy, I'd consider taking about SIX feet out of the boat for poking around marshes.

The kick-up rear fin that is used for turning is a rudder. It will not have a significant effect on the kayak's stability. It will make a longer boat easier to turn.

Raising the bow and stern (adding more curve to the bottom) is a matter of adding "rocker" to the kayak. This is one of the more nebulous areas of kayak design. More rocker tends to make a kayak easier to turn. For poking around marshes this is probably a good thing. Adding too much rocker will lessen the kayak's straight-line tracking. A rudder may help to compensate for this, as may a skeg, which is a non-steerable fin near the rear of the kayak.

Let us know how you progress with this boat; I've often toyed with the idea of applying sharpie shape and construction to a kayak. Conventional wisdom says that a flat bottom kayak is not a good rough water solution, but Chewonki doesn't seem to be losing a lot of kids in these boats.