More mass means more drag ? If "flying", then yes (because you need forward motion to provide lift), but for a neutrally-buoyant device ... are you sure on that?
As Simone points out from his rock-climbing experience (also commonly indicated from procedures for "open water" SCUBA diving when the diver is on a base-fed line, be it a guide line, or an umbilical for air/ mixed gas/ communications/ hot water supply ; I've not done that sort of diving myself, so my knowledge is purely from RTFM), more than a (surprisingly small) number of corners can make it impossible for a climber on rock (let alone a battery-powered set of props) to make progress. More importantly, before you get to that point, the "tender" who is spooling out and reeling in the guide line / umbilical can't "feel" the action at the sharp end of the rope. Which is followed by entanglements as day follows night.
In the cave-diving world, the limitations of "base-fed" line were realised in the mid-1930s, though little was successfully done about it until the re-formation of the Cave Diving Group after WW2. Various contraptions were tried (consult Martyn Farr's "The Darkness Beckons" for historical detail - under "AFLOLAUN"), ending with the present system of carrying line spooled onto a frame with a carrying handle. As you explore into new territory, you spool out the line along and behind you ; if you retreat, then you spool the line back onto the reel as you come out, keeping the line under a small tension to prevent spaghetti-fication. And you pop back out onto the surface, or where you tied you line onto existing line. (In this ROV context, you'd be back at base station, on "surface".)
David and Simone's comments suggest the different arenas of interest for the ROV - exploring spaces too compact for a human (or too unsafe, or too unhealthy - I don't believe Stumpy's tales of concrete diving, but I know people who have had to do multiple-corpse recovery jobs where the water got unbelievably noxious after the first couple of months), as opposed to co-diving with people. I would suggest that base-fed might well be appropriate for the latter case, but spooled-line (with the spool on the ROV) for the former cases.
Driving the ROV and managing the tether in one set of hands? As David indicates, that's a recipe for trouble.
Base-fed tether is conceptually and constructionally simple - breed a humanoid handler and hit it until it stops making mistakes - maybe provide a couple of tent-pegs to assist line management on surface.
What is passing through my mind for ROV-deployed line would be a bolt-on module for the main ROV, which contains it's own batteries, motors and control circuitry. (That reduces the need for co-development between spool and main ROV.)
- When the ROV is progressing forward or stationary, the line spools OUT against a brake (some motors will do that when connected across a resistance? IANAEE - I Am Not An Electrical Engineer.).
- When the ROV is reversing (how does that information get into the spool module? Coloured LEDS through the transparent pressure vessel walls perhaps?) the motor turns to spool IN the line and attempts to maintain a constant tension (a few newtons only would suffice) on the line. If the thrusters are being effective, the motor will run faster ; if the machine experiences drag (walls, current), then the spool motor (and it's batteries) is assisting the ROV thrusters (and their batteries) to bring the ROV out.
- If the spool module fails (flat batteries ; other major failure mode), it should fail "safe", so that the machine can still be hauled in as if on a base-fed line.
I'm trying to work out how to keep the cable count down. Are the FO lines rotatable? Can they be terminated to the inside of the line spool so that the signal gets through without imparting twist to the cables or connectors? If you can do that, then the well-established trick of threading a communication line through the middle of the navigation line (reducing cable-in-the-water count and protecting the comms line from the worst of abuses)) would work. But with a 100m line reel (of approx 4mm line ; not incomparable to the FO lines that I've seen), you're looking at some 200 turns on a 15cm radius by 10cm wide spool (remembering from my caving equipment). That's unfeasable for a twisting connection. I think.
(Why 100m? That's the nominal depth limit for the current OpenROV designs. My brief searches indicate GBP 41 for a 30m TOSLINK cable from a company called "LINDY." But I know next to nothing about audio, so that means very little. These are audio cables?)
I'm going to show my unfamiliarity with FO equipment now : what I've seen in the past has been twin pairs of FO lines, but I don't know why (and never needed to ask). Are separate TX and RX lines essential, or can you TX and RX on a single line by some sort of interleaving of packets/ commands?
As a plus ... given a design for a line-management module that can go into the water ... by tying it to the boat, you've got a line-management unit for a single person at surface to both steer AND line-manage the ROV. I think that sounds like two birds with one stone, which as a geologist I like!
Oh, it needs to be said (people have died for forgetting this!) : tie off the end of the line on the spool, otherwise you'll run off the spool at some point. And people have forgotten to tie the base-fed line to something solid too, in the past. Ask at the rock-climbing club.