I confirm, with my nex6, the 10-18 7"dome port and len is buoyant about 1 kg in salt water with one D1 strobe, baseplate and arms. Also the big dome rotate the camera dome-up and it is not so comfortable to counter rotate the set with your wrist. I counterbalance it with a lead wristband for gym (1 kg) around the base of the dome, but the rotating momentum still exists a little.
Better is to counterbalance the dome with many tiny tyre leads on the dome's hood, I see someone on Web that done it.
The best is to use a glass dome (Zen), but is costly.