the cable you linked to has a rating of 125V peak.. you should not use this with the voltages youre talking about.
Dont focus on CAT5 if youre not neccesarily are transfering digital signals, no reason for a tvisted cable for power transfer. the power-line modem shouldnt need tvisted pair either(correct me if i'm wrong)
look rather for a cable with higher voltage specification, both LAPP and other companies have cables with higher voltage specs.
look at cables with PUR or rubber outer sheeting. pvc cables are hard to watertight.
PUR outer sheeting is fairly good to mould on connectors or in passtroughs.
regarding the 380V converter: is this AC or DC voltage? i guess DC since you want to run a Vicor module. are you aware of the cost of these? they are not cheap here in norway atleast...
i recently did a test at work with transfering 375VDC to a Vicor Maxi 600W brick trough 5km of 0.75mm2 cable.
with this system the voltage decreased to under 250V(minimum voltage of the brick) when the load increased over 110Watt.
on the system i tested on i also tried with 3-phase 400VAC/400Hz(from a inverter(VFD)) into a set of 400/260VAC Auto-transformers at the end of the cable, then rectified and smoothed the power before the Vicor module.
this ended up at 250Watt before the voltage dropped to low.
Note that with AC going on a long cable the power system topside will see the cable as a capacitor, this will cause a rather high current going, unless you compensate the capacitance with a drossel that makes the current go down. if you dont compensate(that is rather tricky to calculate to match) alot of power will be eaten up by the cable.
The strangest thing about a system like this, is that when the load subsea increase, the current on the cable is rather constant, and can actually decrease when the load increases!!
with a cable of 100meter or in that visinity you wont have any big issues with the cable capacitance.
what lengths of wire are you planning?
final note: DONT think about playing with voltages above 50V unless you know what youre doing and are a professional within working with voltages this high. THIS IS DANGEROUS!
"high voltage" connectors for high pressure: http://www.teledyneimpulse.com/ http://www.subconn.com/ http://seaconworldwide.com/