Hi Sarita,

I can second the YSI suggestion. We have been using their CastAway CTD and
it has performed beautifully (~US$5000). It is limited to 100 m depth but
that may be more than enough for your application. The other instrument we
use is also YSI, the 6600 multiprobe sonde. This gives us chlorophyll, pH
and DO as well as CTD down to 200 m but it is a much pricier system
(US$24,000 with 200 m cable, 6 sensors, hand operated winch, data recorder
and calibration standards). [...]