I apologize for the length, but here's something I wrote a while back:
In comparing underwater communications, each system has its advantages and disadvantages. Mission requirement would and should dictate which system is best for the specific operation. Here is a comparison of each system:
With hardwire you have full duplex communication. Not to be confused with VOX. VOX is Voice Operated Transmission (X is an abbreviation for trans/mission-ceiver-mitter, i.e.: xceiver) used as an option with some wireless communications. With VOX your voice keys the microphone. Not the best system when the fertilizer hits the fan. For you firefighters or peace officers, imagine a distressed partner keying the mic of his Motorola HT2000 and not letting off. This is what can happen when a panicked diver goes audible, effectively shutting down communications for all involved. VOX is only half duplex, meaning only one person can talk at any given time.
PTT or Push To Talk is of course the same way (half duplex), however it takes the deliberate act to talk. Most find PTT a better way to control communications while using through water (wireless) comms. However, with four wire (hardwire) system, the tender and diver are able to talk at the same time just as on a telephone. The diver can be monitored through out the dive. If he/she starts to stress, the Tender can hear the breathing rate, grunts, groans - everything. The constant voice of the tender can be very reassuring. In a panic, a diver may not remember to push to talk. This also gets into training. Train beyond panic.
Another advantage for hardwire is control. With the ComRope, the Tender not only has bubbles, but also can estimate distance, depth and direction along with the constant communications. With a deployment, the Safety Diver knows where to find the Primary Diver - at the end of the line.
The main disadvantages of a hardwire system are diving in a snag prone environment and limited range, the length of the rope. The wireless system will allow a diver to swim freely through pilings, trees and other hazards. This is something that has to be thought through from a safety standpoint. Determine if this is a feasible method of diving for your environment and teams level of training. Also take into consideration the drag of the ComRope. There can be a lot of drag with a 150' piece of rope. Having said this, it may be even more important to have a diver tethered on a ComRope if operating in a snag prone environment. If the diver becomes snagged or entangled to a point that self-resolution becomes difficult, how are you, as a safety diver going to find the diver down? Without a tether of some type, you have to start a search pattern just to find your diver? This is not efficient, effective or safe.
Through water communications systems are very reliable. Most of the difficulties are in training and application of the equipment. I find that the teams that use any, and this goes for hardwire as well, of the systems regularly are more proficient in their use and seem to have a lot less problems. The biggest problem is not taking the time to learn how to effectively use the equipment. Things such as microphone position, transducer placement and just talking slow and clearly have an effect on how well the systems works.
With through water communications there are factors and conditions that can affect the communications, both negatively and positively. The signal used for transmitting the communications is ultrasound. Same as a sonar signal, this is nothing but high frequency sound. The technology depends on the incompressibility of water to transmit the signal. They are, for the most part, line of sight; pardon the expression for dark water/limited visibility. They don’t work well around objects or corners. Kelp, algae and plankton blooms can adversely affect the range of the communications. Ambient noise can interfere with the signal as well. A good analogy is that of a large conference hall. Take an empty hall with no people or objects in it and you can hear someone talk at a normal voice from most of the way to the other side. Now, fill this same hall with people, displays, carpet the floor and you’re hard pressed to hear someone from 20 feet away.
Thermoclines can have a dramatic effect on through water communications. Think of a thermocline as a reflective layer, the more oblique of an angle the better the reflection, just as skipping a stone. With a thermocline, it’s best to talk as close to directly overhead of your divers from a surface station or to get the transducer into the same thermal layer. Diver to diver, as well, it’s best to be in the same thermal layer. I’ve seen a thermocline completely break divers from a hundred or so yards from each other, where within the same layer, the communications were loud and clear. These same thermoclines can act as a medium of transmission, like yelling down a pipe. For maximum range, have a calm surface and a dense thermocline and a wireless signal will scream. Depending on the equipment, you can get ranges of 400 meters to in excess of 6000 meters under good conditions. This certainly has its advantages if your mission requires range.
Each type of system has its advantages and disadvantages. Examine the type of diving you do, determine the best equipment and learn how to use it. As with any discipline, you can have all the gear, but it gets down to the training.
Dive safe...
my my what a knife and a little aqua seal can do 
