Yes. I would say for the same reasons as for fish, you need some currents. If the water is too stagnant, the particles will not go toward the shellfish and the seaweeds.
Also, the positioning is very important. There is not one design. That's where the work of people such as Fred Page, physical oceanographer at St. Andrews Biological Station, is very important in tracking the movement of the organic nutrients. That information helps to determine different placements of the mussel rafts and the seaweed rafts. So there is not one design; it has to be adapted.