Scupper Cart
This is a scupper cart. It is a set of wheels that fits in the scupper holes of the kayak and lets one person wheel it around like a wagon. The big advantage of this sort of cart is that when you get to the water, you can pull it out, flip it over, and stick it in the top of the kayak. Then off you go - no need to double back to put the cart away. Also, you'll have it if you get somewhere that you might want to portage.
This one says West Marine on it, but it is the same as a 'Scupper Swift', which is what I have an older model of. It is made from aluminum bicycle tubing. Years of saltwater have not affected it. I had to modify mine to fit my particular kayak which is non-standard. It will fit most kayaks right out of the box. Comes with a lightweight tiedown strap so you can secure it. There are many similar models on the market now; I would recommend that you go with the most compact model you can find, so you can do as I do and take it along on the water when you want to.
If you have a 'sit-inside' kayak, then you'll need to drill a set of holes in the bottom to use one of these carts.