Two places that would have changes from the usual human structure are the ear and brain. With just a few alterations to these organs and you could have a humanoid creature that could use echolocation effectively enough to "see" under water
Dolphins can produce clicks with their vocal cords; your humanoid should be able produce a similar sound. If not, some slight change of their vocal cords should make it possible.
It is possible for the human brain to learn to interpret sound as sight. However, the human brain is not set up to be used this way and can't do as effectively as a dolphin. Dolphin brain is optimized for interpreting sound into sight. Your humanoids would probably have a brain that was a hybrid between a human brain and dolphins.
Other changes would be in the ear. The Bottle-nose Dolphin hears tones with a frequency up to 160 kHz with the greatest sensitivity ranging from 40 to 100 kHz. The average hearing range for humans is about 0.02 to 20 kHz. So your humanoids' echolocation would much more efficient if their ear structure was more like a dolphin then a human.
specifically changes you should make would be to the mechanical stiffness of the middle ear and the cochlear input impedance. These changes would alter the middle-ear transfer function. The dolphin ear also has some-filled sinus pockets that allow for greater directional hearing underwater.