Not really, assuming the water isn't that deep you could wade out and harvest that way. Plenty of food plants are grown and harvested in water like this.
From the looks of it there is a considerable outer layer of lighter plants that are likely required to have a good plant-mass to flotation device ratio, but this layer makes it hard to access the inner plants that could be used as food crops.
Yes, but to "harvest" such plants they just need to pull them out of the water and compost them somewhere. Not quite the same as harvesting some actual food product.
Living in an agricultural community where several of the ranches and farms have artificial ponds, I almost wonder how well this would work there. Extra plants and harvest but also the shade created by the plants in theory may help combat some of the evaporation.
That reminds me of Chinmapa from Mexico! A chinampa is a floating garden built on a freshwater lake, made by making a raft of woven reeds, with stakes to keep it in place. Soil was placed down until it was above the waters surface.
This system eliminates the need for watering, and was successful in growing maize, fruit, tomatoes, amaranth, beans, chile, and flowers.