No worries - good ideas thrive with attention 
Very glad you’ve got some relevant electrical training and experience
Always scary when people are playing with HV without knowing what they’re doing - we’re trying to grow the underwater robotics community, not shrink it…
Just confirming, is this “means of power” (for topside electronics) and “telemetry to the ROV”, or are you trying to supply power to the ROV from the topside? I’m assuming the former, but if it does happen to be the latter then your system is likely quite underpowered, and there are some missing/undiscussed components with respect to power transfer and conversion down to usable voltage levels within the ROV. If it’s relevant/you happen to be interested, there’s a relatively in-depth breakdown of power transfer requirements to the ROV and conversion efficiencies in this comment.
Nice! I wonder whether it’s worth making the platform retractable, so it sits at the desired height when in use, and then slides down flat when you want to pack away the box
Seems reasonable
If it’s liable to being kicked or knocked it may be worth including some kind of protective cylinder or something around the connector to help prevent accidental (and potentially damaging) disconnections.
Keen for updates and to see where you take this design process, hope you’re able to return safely soon