You can also make your own Aluminum dock (or Aluminum I beam) docks at a fraction of the price of buying a "package" deck.
Aluma Beam ..... it even comes with a poly insert in the top pocket so you screw wood (or composite) decking into the Aluminum beam without having to drill into the beam itself. It also comes in various lengths and can be purchased used as well.
If you flip it upside down - it has a channel slot to slide in a bolt from the end, so, again, you can fasten down decking (or other pieces) without drilling any holes in it. Just slide the bolt head (or nut if you are going from the top) into the channel and down to where you need it and you are set ....
This makes joining each section of dock with a shared top plate a breeze - in and out in minutes per section each spring and fall.
The product is used for concrete forming and shoring of structures - so it is far stronger than you can imagine, and far lighter. 3 sticks weighing a total of 48 lbs each will hold up a 1/2 ton truck.
It used to go up and down fast on job sites, easy to use, versatile, and light and strong.
If I was doing it - this is what I'd be using if I was making my own.
https://gatewayindustriesonline.com/...ms-Joists.html
The brand name you want is ALUMA BEAM - there are a few knock offs as well.