As I indicated, detector selection depends exclusively on the dynamic range and ultimate sensitivity required. You've given almost no information about the path length or amount of power involved. This leaves open using anything from PIN detectors to photomulitplier tubes.
There is also some question as to the validity of using a laser, since the scattering coefficient for water is extremely high. This means that probably 99% of the transmitted energy will not reach any detector you could possibly configure; is that allowed?
Transmission is a very non-trivial measurement and you should get someone a little more knowledgeable to ensure that you're doing it right and doing the right thing in general.
TTFN