Hi @Henkedoodle -
Welcome to the forums!
This is a common topic - have you read some of the related threads?
Blue Robotics sells the Outland system for this application. Other vendors have solutions too!
The critical thing to remember is that AC (and DC) voltages, especially above ~30V, are INCREDIBLY dangerous! Any power over tether system needs to have proper fault-interruption circuits - using a household “GFI” is not enough - a line insulation monitor of some kind is typically used- I’ve tested with models from Bender. Vicor makes good DCDC power converters - I would avoid AC on the tether.
A simpler idea may be to keep your ROV battery power, but charge the battery at a low rate via a DC surface power supply. You still need to take care to keep the battery cells balanced, but this can be a lower budget solution that is not only safer, but cheaper to implement. The voltage drop calculator is relevant, as is this new guide!