If you have used a smaller waterproof camera in the past, particularly one where the flash is built in to the body of the camera, you will probably have been a victim of something known as backscatter.
It is one of the biggest problems when you use an underwater strobe.
It occurs because almost all water contains small particles of other matter.
If you are diving, this can be a real problem to visibility at times but even in clear-looking water, there will some element of this problem.
It is very rare to find water that is completely clear from this problem.
The trouble arises because all that "stuff" tends to bounce light back in the direction it has come from - literally scattering it backwards, hence the name backscatter.
If your strobe just happens to be near your lens, then it all gets bounced straight back in, stealing the light from your subject and adding a bright sheen to the whole image.
The way round it is to get the strobe separated from the camera in some way, that way the light bouncing back to the flash isn't coming towards your lens.
For this, you should look at the range of extension arms available.
Some provide a fixed, lockable support, others a more flexible series of joints.
If you really can't do this, try just using the very edge of the strobe light cone on your subject, it may help - again because the majority of the bounced light is going somewhere else.
You just may need to up the power on the strobe with this option.