Question

what is the wavelength range with which bacteria under water absorb?

Answer 1

It depends on the clarity of the water, in pure water, nothing as if you put bacterial cells in pure water they will die. In ocean water or pond water, it again depends on the clarity and "color" of the water.
I'm assuming you mean bacteria that absorb light as not all bacteria do, so a "correct" answer to this question could be "nothing as you are making the blanket statement that all bacteria absorb light and that is entirely not true". I imagine whoever you are answering will not like this answer however.
A better answer would be to assume that these bacterial cells absorb the same sort of light that chloraplasts do and this would be blue light and red light.
Or if you REALLY want to make this person hate you, read this wikipedia then base your answer off that! http://en.wikipedia.org/wiki/Photosynthetically_active_radiation

Answer 2

the question needs rephrasing in order to be given a more concise and precise answer

Answer 3

Bacteria underwater that absorb light? Sounds like the question is probably asking about cyanobacteria. What you need to do is look up what pigments they have (chlorophyll, etc.) and wavelengths those various pigments can absorb.