There are very long floating piers built in the saltwater bays around San Diego. They have concrete posts that go thru the pier, and there is a roller attached to the pier for every flat surface that is on the post. Because of the stiff concrete posts, they cannot sway from side to side. The piers are at least 6' wide IIRC. Wide enough for a fish cart to pass another fish cart on the dock. I estimate that some of those carts had at least 1,000# of weight in them, either taking supplies to the boats, or offloading fish.
IIRC, a member built a floating dock and didn't have a very wide "T" section at the end. They said that it was very rocky.
Generally, the wider the pier, the less rocking will happen. Lateral structures coming off the main pier at 90° angles will also help reduce the rocking provided they are anchored very tightly to the main pier to eliminate any movement at tht 90° junction. Piers with more flotation will sway up and down less than a pier with less flotation.
If you have unlimited latitude in placing the pier in the pond, it will have the least movement if placed on the lee side of the pond. (the side from which the prevailing winds come from). A head or tail wind will move a pier less than a cross wind.
I am not going to drive any type of vehicle on the pier, and my floating portion of the pier will be 4' wide, by 16' long. At the end of that 16' section, there will be a 12' wide, 8' long section. Think of a "T" with a fat top that is 12' wide.
If you look on
this page, those tubes going from the end of the dock to shore, and the guy wires criss crossed are there to stop the dock from swaying side to side. I'm assuming from boat wakes.
