Cottage Life posted an article on how to build a floating dock. The debate between plastic and foam docks are considered. The article considered the benefits and challenges that come with both plastic and foam.
“While there is much debate about the relative merits of foam vs. plastic, closed-cell extruded polystyrene foam is widely accepted as better than EPS which, over time, wicks up water (and possibly oil) and can lose buoyancy. Billets should be snugly encased in wire mesh and/or wood to protect them from gnawing wildlife, which will nibble the underside of your dock and spit out little pieces that end up littering the water. This is not good for the health of animals or the lake.”
There are other more modern options outside of simply foam or plastic. There are more recent foam float docks that are rubber filled with foam. There are several benefits to these more modern float docks. These benefits can be seen here.
Read more about the plastic/foam debate here.