Buoyancy control using pump / valve / bladder is very slow to respond, but may be beneficial in certain applications. (In my opinion)
Halogen lamps are power hungry (1.2 A for 12V lamp), use LED clusters instead, 200mA for 7 led's.
Wireless camera's or wireless data transceivers @ 430 Mhz or Bluetooth signals do not transmit through water.
Ensure area is free from blanket weed, it causes havoc with the props.
To prevent lake bottom debris from fouling vertical thruster prop mount it as far from bottom of ROV as possible.
Updates
Tested RS232 communication via a 54 metre cat 5 cable from StampPlot GUI to the BS2 microprocessor in the ROV and all controls operate fine.
New camera with 30 infrared led's fitted. Tested longer comms cable / cable drum / slip rings by making another video.
Communication has started to fail. RS232 to RS485 converters on order to rectify the problem.