How to measure water turbidity with arduino
http://uk.farnell.com/ams/tsl257-lf/sensor-light-voltage-converter/dp/1226886
is a good one according to http://forum.arduino.cc/index.php?topic=299993.0

But it might be easier to buy a COTS sensor, its a bit more expensive but much easier.  http://www.aliexpress.com/item/Turbidity-Testing-Sensor-Module-Suite/1903197650.html?spm=2114.01010208.3.17.OSVwdj&amp;ws_ab_test=searchweb201556_9,searchweb201602_2_10017_9868_10005_10006_10034_10021_507_10022_10020_1000020_10018_10019,searchweb201603_8&amp;btsid=6dbcca61-3e0a-4e34-9359-a6c26b268509

BTW for everyone else Turbidity is the amount of muck in the water, so it would be no good looking at the crystal clear waters off the UK coast :)