One of the most important issues to monitor on an OpenROV dive is leakage. For obvious reasons, you don't want water seeping into the electronics tube. The o-rings on the end caps provide the seal, but it's nearly impossible to visually verify if the seal is sufficiently watertight, even with the clear, acrylic tube.
Of course, the best defense and leakage is preparation. Around the lab, we've started using a Mityvac to test the seals. We attach the suction-cup-type piece to the syringe valve (with the plunger removed) and pull out some air. If the gauge stays put at the new air pressure, we know we have a good seal.
I'll post a video of this being demonstrated in the next few days.
The Mityvac is available from Amazon.