Rick:
You can get Durham's Water Putty at Lowe's. For some goofy reason, it is kept in the paint aisle near the solvents, not with the wood fillers. One huge caution. I used Durham's on some of the same types of gaps that you mention. It mixed well and was easy to work with, but it dries (undyed) to a uniform bone white. The uniformity of the material makes it pretty obvious. It also settles into any imperfection in the wood and increases the sanding job. I have since removed a good bit of it and replaced it with an epoxy/cab-o-sil/wood flour blend. It seemed to blend much better with the surrounding wood and was more easily matched by blending wood flour from my sander's dust bag and cab-o-sil to lighten.

If you have a crack where you can see daylight, you can whittle a splinter from a matching strip and glue it in. If the fit is tight, flatten the piece with a pair of pliers. The glue will make it swell back up. I even used shavings from my plane to fill thin gaps.

Unfortunately, for every gap that disappears under the plane, another pops up. As you can tell, I have done quite a bit of filling on my Great Auk, but on the recently glassed hull I cannot find any of them, even when I know where they are.

Regards,

Marcel