Hi @eivinmb -
Welcome to the forums!
What you’re proposing to do is EXTREMELY DANGEROUS! Whatever method you use (AC is more dangerous than DC), you’ll need to use a line isolation monitor to make sure that damage to your tether immediately cuts power to the subsea system, so that nothing gets electrocuted!
The Outland power over tether system is the Blue Robotics recommended solution, as it has this protection built in. However, other Blue Robotics distributors and groups make and sell alternative power over tether systems. Some users have had success with DIY. Be extremely careful!