Theoretical maximum is 50m/165’, as per the technical details, but that depends on target size (a single ping would need to hit a 22x1.75m/73x5.75’ target to be fully reflected at that distance), and gets reduced by bubbles and other floaties in the water scattering and absorbing the sound, along with the composition of the water and the frequency of the sound.
Out of interest, were you testing on high gain, or did you leave it on normal? It’s also worth noting that while the receiver is tuned to 750kHz, lower frequencies attenuate less so you may be able to get a little more distance by reducing the transmission frequency a bit (I’m not sure what drops off faster between the receiver sensitivity and the water transmittance). It might be worth doing some testing to map out the maximum achievable distance across different water types and transmission frequencies - perhaps a project for the bluerobotics engineers, or an interested third party 
This is where more cameras, proximity sensors, and/or live 3D mapping come in handy. Shame all of those are a bit of a pain to set up/get connected, but so it goes. Can definitely recommend a rotatable camera on top if you’re regularly going into tight places and have the penetrators and ports to connect it - this comment may be helpful to sort out the software side if you’re interested
I think you succeeded! My speakers were up a bit louder than I realised and I felt like my head was underwater 