the sub is positively bouyant, and so floats on the surface when not under any kind of power, and to keep her down, the vertical axis motor has to be regularly blipped. In the you tube vid, you will see her starting to rise the second the momentum generated by the dive thruster stops, which is about an inch of depth dived after the motors stop
To turn off the LED's may save me a second or two in the dive, they pulll naff all current, as regards the weight of the V, it is for that reason that I may physically tether her, so if she goes down I can retrive her by winching her back aboard, very much like a surface controlled ROV such as the Scorpio is brought back, that way it doesn't matter how heavy she is, want to go down, winch out, want to go up, winch in cable.