Circuits inside high-pressure oil
Your biggest problem will be with sealed air spaces inside components. I know that EPROMS were commonly regarded as likely to fail under high pressure. More up-to-date examples would be quartz crystals and oscillator modules. Power transistors in TO-3 cans would have an enclosed space, as would some kinds of small transistors in metal cans (or op-amps in metal cans, if you remember those).