I have a Chesapeake LT17. The back edge of the cockpit is just an inch forward of the bulkhead. That bulkhead can take a whole lot more than your weight. Don't worry about it. Now don't start walking around on the other parts of the deck, they are not designed for that kind of loading at all.

I personally find it much easier to: From a standing along side in shallow water with the boat afloat and paddle in one hand to slip one leg all the way in and sit down into the seat all in one motion, then bring the other leg up and in. This bypasses the sit on the coaming altogether. The problem with the "sit on the coaming" idea is for bigger people (I'm 185), the weight on the kayak up that high makes it VERY unstable. It's not a strength issue, it's a stability problem.

Craig