The key pieces of rowing equipment include far more than a boat and oars today.Long ago a coxswain might have had a simple cone to focus his or her voice and they might talk into the bottom of the boat in hopes that it would carry their voice to the rest of the crew. Today as the coach on the water the coxwain is an integral part of any crew and the cox box and speakers that ensure precise communications with the rowers is no longer a nicety - it is an essential piece of equipment.
Cox boxes give stroke rate, stroke count and time as well to help the coxswain guide the crew in the perfect execution of a race plan. This of course with its most basic function of voice amplification carrying the coxie's voice to 2-3 or more speakers placed in the boat near the crew.
Nielsen Kellerman have produced the almost de facto standard - so de facto that their name "cox box" is the standard like Kleenex now means tissue to all or rollerblade means in-line skate.
It would be hard for anyone to dethrone the leading company - so many clubs have multiple units and training coxies to use a different brand or even to risk trying another brand with a relatively expensive purchase.
Still, others are trying to bring their game.
NK units don't give boat velocity and supply limited feedback to the coach after a workout or race. Some clubs combine the cox box with another piece of equipment like a speedcoach to get velocity (there is no consensus if these units really are accurate with bigger boats).
A company like Coxmate is worth a look. They produce several units - the most sophisticated being the SCT which handles the basic functions but adds velocity with a micro-impeller that is significantly smaller than the NK unit or even a GPS function. It also has a PC link for coach analysis.
in2Rowing made a unit packed with features, including GPS but their web site now says they are shut down "indefinitely."