I'm working on an ROV project as a senior design project and am nearing completion of it. The current plan house all of the electronics (arduino, motor drivers, telemetry) in a polycarbonate box filled with tech grade mineral oil. The mineral oil does two things for us: (1) equalizes ambient (external) and internal pressures (2) protects electronics from humidity/moisture. By equalizing pressures we can easily keep water out.
Anyone see any issues with submerging electronics of this nature in mineral oil? Max depth the ROV will achiever is 30' so capacitors should be fine as far as pressures go.