When we did the bathtub tests with our OpenROV, using cold water, the inside of the electronics tube quickly fogged up, which makes sense since the warm air around us when we sealed it up holds more water than cold air.
To avoid this, I did some quick research about possible desiccants, and found this page: http://www.backdoorsurvival.com/beginners-guide-to-desiccants where calcium chloride was the clear winner.
So I bought some calcium chloride (it's apparently used in pickling!), put a bit (about half a teaspoon) inside a paper towel "pocket" and then thoroughly taped it shut, and put that home-made desiccant inside the electronics tube, sealed it, turned on electronics (so it heats up the air inside), released the air pressure using the plungers, until that reached steady state, then left it sealed over night so the desiccant had a chance to absorb all humidity.
Then, we sailed it the next day in our local pond and there was absolutely no fogging up!
I was a little worried about putting a salt inside there; if there is a catastrophic leak it will turn the fresh pond water into salt water ... but so far it seems to be working well.