For sure you can build a reinforcing concrete floating dock or a Ferro-cement one. I think for the conditions you shortly described, there is more appropriated a reinforced concrete than a Ferro-cement dock.
 The pontoon you are requiring, I guess, you can calculate starting from the more simple buoyancy formulas. As a matter of fact this is not a ship destined for navigation, but an intermediate  devise serving you to face season water levels.
In the American continent you can find several examples of concrete pontoons, but in the sub-tropical strip of the Americas, you could ask Cuban Marine Construction Enterprise.
I heard that they built several flat top cargo barges of reinforced concrete and the designs were approved even for the Shipping Register, so these pontoons are able to sail in the Caribbean.
I think there they have the experience that could help you