Most any hiking compass can be used for underwater survey as long as it is liquid filled.  Suitable compasses can be found for as little as $10.00, but better accuracy and ease of use can be had for a little extra money.  For accurate survey, a compass marked in two degree increments, and a rotating bezel are essential.  Adjustable declination is also very convenient and saves having to correct readings later.  For surveying caves, a luminous bezel facilitates taking readings and adjusting the bezel without having to shine a light on it every time.

The Suunto A30 compass is the one I recommend and has all the features listed above. They cost around $20. The main difference is the Suunto has a luminous bezel. Unless you have a good, head-mounted light source, it is difficult to take good azimuths without the luminous bezel. 

Below is a list of compasses I consider suitable for surveying underwater.  What most of them lack is the luminous bezel.  Links to the manufacturers are at the bottom of this page.:

Silva Polaris Model 177

Silva Explorer Model 203 (Luminous Points)

Brunton 3DLU

Brunton 8010G

Brunton 9020G

Brunton 7DNL

Suunto A-10

Suunto A 30 L

Suunto A40L

Suunto M2D

Suunto M-3DL

Suunto M-5DL

Personally I've used the Suunto A30, Silva Polaris, Explorer, and Suunto M2D with good success.

One other factor in compass selection is compass "dip".  The earths magnetic field not only varies horizontally as "declination", but also vertically causing the needle to point downward when the case is held level.  Compasses are balanced with a small weight to compensate for the dip. Otherwise, the needle could hang up in the housing causing incorrect readings.

The earth is divided into five zones by compass manufacturers to compensate for dip.  The northern hemisphere is pretty much all zone 1, a band containing the Caribbean is zone 2, the equator are is zone 3, South America is zone 4 and the Antarctic is zone 5.  Some compasses are balanced for several zones, others are built for specific zones.

I've used standard North American compasses in the Caribbean and Mexico and didn't have any problems, but that is the extent of my experience.  If you plan to take a compass into another zone, it would be wise to check the suitability of you compass for use there.

Brunton offers some excellent compasses.  More information at the company website. 

http://www.brunton.com/catalog.php?subcat=26

Silva carries a wide line of quality compasses and they can be found at: http://www.silva.se/outdoor/products/comp_start.htm

Suunto has great compasses for underwater survey and  can be found at: http://www.suunto.com/suunto/main/~index.jsp