We do not think you will have any issues measuring in the reservoir provided there is good scattering throughout the water column. You can look at beam velocities in post processing to verify the horizontal homogeneity assumption, but we have found this generally holds except for exceptionally weak velocities. Unfortunately, we cannot provide better accuracy numbers, but our experience has been in most situations the uncertainty in measurements is dominated by the random component. Low velocities are always a little harder to deal with, but we do not think you will have any problems.
We definitely recommend having a temperature profile nearby though. Sometimes the thermocline can be tracked from the amplitude data, but sometimes there is not a large difference in scattering there.