I've seen lots of home-built docks. They are popular because they are affordable. You basically build a deck and put some styrofoam blocks under it so it will float. Another variation is the sealed empty oil drum for use as a float under the dock. But the oil drum docks are too high for easy kayak entry. Nice thing about the home-built dock is that you can make it the right height.

If you want to go with commercial docks, the "jet dock" system is pretty neat. Interlocking floating plastic blocks offer a lot of variations on the setup. Alot like legos. My college had a long one for our crew team. Unfortunately you need to be able to afford one. I think the docks cost $20-$30 per square foot. You need hundreds of blocks for a dock of any real size. It makes a real nice dock though. It can be assembled by one person, piece by piece. You can see their stuff at www.jetdock.com and in the image below. (In the picture below, the blocks are thicker than I remember. They might be a little too far from the water's surface.)

Some outfits will also build a dock for you, but I don't know which ones might be in your area. Aluminum seems to be popular with many of them.

My dad built a dock out of wide aluminum strips that had been bent for strength and riveted together. It has styrofoam for floatation. Very light and strong.

But as Chip said above, a beach is best.

Good luck. Sorry I can't give you better info.