Many people forget the biggest problem with atmospheric housings is not the pressure, it's the "differential" pressure. Any housing will have a psi rating for which it will "collapse" or "implode." One simple and inexpensive way to increase this depth rating is to pressure the atmospheric housing up with a compressed gas while the ROV is on the deck.
For example, the psi of water pressure pushing on the atm housing at 100m of water depth is about 160.5 psi. Assume the crush pressure of the housing to be around 200 psi. Now if you were to pressurize the housing with air or even better dry nitrogen to about 90 psi before you started the dive, you would effectively add an additional 50m of depth capability to your ROV. (50m water depth is about 87.5 psi) When the ROV reaches about 50m of depth, the internal gas pressure would equal the external ambient pressure.
There are concerns in using this method though as you have to ensure you do not have any "hollow" electrical components that might implode at the ~90 psi over pressure. Also, you have to ensure your "seals" on the atmospheric housing work in an outward force as well as an inward, which most do not.
I just found this site today, thanks to kick starter. I hope to build my own OpenROV (actually 3, one for myself and one for each of my children who are 11 and 10.) I will be around frequently until I depart for the South Pacific for work in a month or two. I plan to work on a power system for this project that will allow you to trickle low current power to the ROV continuously, and use the power as needed. :-D Worked on a similar project for one of the companies I worked for, but the project got canned because they didn't need the technology anymore. Granted, that was to run a 200hp ROV so it was a bit more complicated. LOL
Glad to be here and look forward to getting to know you all.