Posts have been sugesting single ended range finding equipment.
The original question was about measuring the distance between two intelligent co-operating devices.
Sound travels well in water so 150ft should not be an issue.
What you need is a device that transmits a sound and another device that detects it.
The sounds should probably be at frequencies that travel well but don't occur in the background, and both devices should use different frequencies (to avoid a sender receiving a reflection or echo of its own signal).
Device A transmits a sound and starts a timer. Device B detects the signal from A and transmits a sound in reply. Device A detects the reply from B and stops the timer. Device A then subtracts a figure from the time to allow for processing, divides by two and, using the speed of sound in water calculates, the distance.
Most of the time I think this would work quite accurately. In special circumstances where A and B are at different depths the sound might be blocked by a boundry in the water. Boundries between hot and cold water (thermoclines) and salty and fresh water (haloclines) can be sharp and visible so I would be surprised if they do not affect sound and perhaps trap it within a layer.