I’ve been building and running work-class ROVs for a couple years, including some with 150-200m tethers. Totally get the headache you’re dealing with.
On the 230 VAC thing: yeah, I tried it first and it sucked. Voltage drop was like 15-20V under full load, efficiency tanked around 75%, and the tether got warm enough I was worried about insulation. Switched to 400 VAC and it was night and day—way less current, minimal drop, almost no heat. If you can swing it, go 400 VAC or even 300-400 VDC. DC is cleaner for long runs if your topside supply supports it.
Power conversion on the ROV: I mount a sealed “power bottle” right on the frame. Took a commercial AC-DC module (Vicor or similar ruggedized one) and potted it in epoxy inside an aluminum housing. Handles 230/400 VAC straight down to stable 15 VDC at 100-200W no sweat. Super reliable underwater. If you want something cheaper and already subsea-rated, hit up some Chinese suppliers—they make off-the-shelf units that bolt on easy.
Custom tethers: I went with a hybrid power+data cable, 4 AWG or thicker copper for the power lines to keep resistance low. Added shielding for data and used proper strain relief. For connectors, I bought from HYSF SUBSEA (Chinese company). Their underwater connectors are legit—IP68, handle 400V easy, no leaks after months of dives. Way cheaper than the big European brands but zero issues on my last two builds. They even helped spec the pinout for my tether.
Lessons: calculate your cable gauge based on total amps (I use an online voltage drop calculator), always add a ground wire, and pressure-test the whole thing before the first dive. Heat is the silent killer—keep power under 70% of cable rating.