Well, I'm no sub expert but I do have one - and a whole pile of boats. Since you haven't gotten any other responses I'll try to help. Most if not all small subs use "dynamic" diving system, ie they have dive fins that are servo operated and dive under power. The sub is weighted & balanced to float at something around mid depth on the hull & level. On these subs forward motion is required to dive & maintain depth. If the motor quits the slowly float back to the surface. Working ballast tanks are (a) expensive and (b) require more room than is available in small subs. When using a working ballast tank system it is EXTREEMLY important to have a failsafe to blow the ballast in the event of radio failure or the sub will sink to the bottom & die there! You can make a simple, cheep, dynamic dive sub from PVC pipe with shaped nose & tail cone. Use brass plates for the forward & aft dive planes. Make a water-tight radio/motor box - you could use tupperware - another watertight compartment forward for the battery. The rest of the hull needs holes so it can fill with water - make sure there is nowher that air can be trapped or it won't flaot leve. lUse a 500 size motor. Make sure the boxes remain water tight! Remember that the water will be UNDER PRESSURE depending on the depth. Unless your diving in very clear water you won't be able to see it!