Being over water simplifies the pipes, but you must be sure that the device floats, this is not so easy, small ships sunk very often and need a lot of maintenance, specially in salty water.
Also, the dome/jellyfish shape is nice, but the light each plant gets is smaller, in a flat surface each plan gets more light. The plants essentially convert sunlight into food, so more sunlight is better.
And most commercial plantation relay in very cheap (or free) water. Any filtration to use salty or polluted water will increase the cost of production.