Hi,
Most probably you have the water without the caustics (Caustics are off), which causes the inner shadows to not be calculated.
The solution, which is to "brighten up" the water, is about mostly understanding how the water model and material are setup (is it a volume or a plane, is it enclosed or not).
As a first thing, you can try enabling caustics for the water and also do not forget to enabled the caustics solver.