Thanks to comments from Jonathan Groff, I have investigated the longevity of the PC-Fan motors underwater. Story made short, 1 Fan lasted 90 minutes while the other fan started showing signs of deterioration after 180 minutes. The main cause was corrosion on the pcb of the fan. It might be possible to insulate these in a ball of grease or something similar. However after taking 1 apart, I decided the effort was futile. Therefore PC-Fan propulsion will be declared a fail (Lesson learned: Dont trust a 5 minute water test).
These fans were chosen for their cost (3$ each) to keep the overall buoy cost down to around (40$-50$). The options left to us are using brushless-dc motors (Used in the OpenROV project) or bilge pumps (Recommended and used by Jonathan Groff). Brushless dc motors, commonly used on multi-copters can cost around 20$ each including the ESC, which will max out the budget for 2 motors. Bilge pumps like the Rule 360 or Rule 500 come in at around 22$ each which is also too expensive for this project (See the ROSIRA for an interesting single bilge pump with solenoids solution that might be a viable option for forward and reverse functionality). However, small submersible pumps available from Banggood.com have been ordered for $2 each which are cheap enough to simply give them a try. They might not be strong enough but that will be tested once they arrive. If this does not work, the slightly more expensive options may need to be implemented.