As a schematic it looks fine. I don't really know why you want to run a ground cable all the way around the boat. All of the devices shown are designed for parallel cables + and -, so you would probably find that splitting them (i.e. running all the -ve wires to ground) would be a lot more like a rat's nest than sticking with convention. If you're intent on "earthing" the model then you could run just one cable from the P92 -ve to one of the prop shafts, as Nemesis says.
BTW don't forget that if you are using the BEC feed from P92 in conjunction with ESCs which also have BEC then you'll need to disable the Red Rx connection in both speed controllers.
As regards interference, the sensible things such as keeping the main power cables (battery to P92, P92 to ESCs, ESCs to motors) as short as possible and well away from the receiver aerial always apply. If you're using brushed motors and a 2G4 radio then you can get away without suppressor capacitors across the brushes, although I'd fit them anyway in case your motors might affect someone else's radio.
DM