Creating a neutrally buoyant tether that is strong yet flexible can involve considerable research of past efforts and development.
The object of this discussion thread is to examine the various ways that a flexible, neutrally buoyant, cost effective tether can be either purchased or constructed at a reasonable cost with “reasonable” being considered a relative descriptor.
I presently have 75 meters of the BR Fathom Tether which is of outstanding quality, however the level of its flexibility has prevented me from being able to conveniently store it on a drum/reel of reasonable size and weight and can require a support person to help with the logistics. For those builders using the Fathom-S Interface this tether is the ideal solution unless one is inclined to build their own multi twisted pair tether.
These two OpenROV twisted pair tethers are designed for bidirectional communication using a Fathom-X like interface. The biggest drawback to the unjacketed single twisted pair tether is that it is not physically robust and is not neutrally buoyant. The neutrally buoyant jacketed twisted pair tether has a Kevlar center thread which makes it physically robust, but very pricey. Both of these tethers are small in diameter and flexible, though they both vary significantly in cost.
To overcome the physical limitations of the unjacketed single twisted pair tether it can be threaded up the center of a 1/4 inch diameter poly rope. Several discussion threads (Tether Management 1 / Tether Management 2) have documented this process. Not only do they document the threading process, but offer tether storage management solutions. Use of the poly rope can approach the neutral buoyancy requirement, but it depends on the characteristics of the poly rope in use.
The OpenROV neutrally buoyant twisted pair tether is the ideal solution as it does not require the cable to be threaded up a poly rope and can be easily stored on a standard cable reel. However this tether is pricey at around $5 per meter and obviously does not allow for additional signal paths.