The cracks along the top of the transom are normal and are showing where the liner comes up and over the top of the transom. You can seal it and as long as it hasn't had water intrusion, should last you a few more years. I've never seen a big crack on the side piece of the transom - if they stopped the wood short of the top I guess that would happen. If you want to use the boat as long as you can, seal it up good.
