It submerges: Four small aquarium pumps move water in and out of the ballast tanks to control buoyancy. A single brushless DC motor, which is naturally waterproof, provides thrust. Two waterproof MG995 servo motors actuate the rudders for yaw and pitch, which are necessary for diving/surfacing and steering.
Reading the Instructable, it does not appear that's the case. Instead, he uses the aquarium pumps as thrusters. There are no ballast tanks. I read through it looking for this particular answer, since I couldn't fathom any way an aquarium pump could fill that role. According to the article, some people do use pumps in conjunction with a snorkel, but obviously that limits your dive depth to the height of the snorkel.
Right, he uses rudders to start the dive and then once submerged can use the aquarium thrusters to rise and sink without having to move forward or backwards. The article does indeed misstate how the aquarium pumps are used.
