Most strobes are rated with a guide number. The guide number is the distance (usually in meters) that the strobe will give effective light. This is generally measured topside. If a strobe is guide #20, this means it will have an effective reach of about 65 feet through air. Underwater is a different story. Water conditions vary, and water is not as good a conductor of light. When you buy a strobe, the higher the guide number the stronger the strobe, if the manufacturers measure in the same way. The other factor is the spread of light. If given, this is usually in degrees, so if you are shooting through a 105 degree wide angle lens, you would want a strobe or strobes that could cover a similar area.
A good example is my Olympus FL-20 strobe. It is rated at guide #20, but has a very narrow beam, vertically. I get a picture that is dark on one side and too bright in a single area. On very close macro shots, it's not much of a problem, but the farther you get from the subject, the more area is underlit. It's like shining a spotlight. So ideally, you're looking for a strobe with a high guide number and a wide spread of light.
Another factor is adjustability. If your strobe only has two settings, say half or full power, you will have to adjust for different lighting conditions by changing the distance from the subject or changing your camera settings to suit the strobe. The more steps or levels of power, the better.
Finally, there is color temperature. Different strobes operate at different color temperatures. I believe sunlight is 12,500. If you have two strobes that are mismatched, different parts of your picture will be different colors.
Matching the firing interval of the strobe to the minimum shutter speed of the camera is also important. The strobe will fire for a shorter period than the shutter is open. Some strobes might not be capable of a short enough burst for some very high speed cameras.
I hope I got all this right, and I hope it helps. If not, I'm sure one of the technical gurus like Ron will pipe in to correct me.
Sorry, I forgot your other question. The easiest way to measure your light is to take a sample picture in that light and adjust your strobe up or down depending on the results. When photographers talk about "bracketing". They are referring to taking multiple pictures of a single subject, possibly with different settings of camera or strobe, or different lighting angles, etc. This allows you to have a choice of pictures of the subject when you are finished. Some will be better than others. Some strobes have automatic settings. You can set the strobe to match your shutter speed and f stop and it will decide how much light it needs based on the sensors in the strobe. Other strobes use "ttl" or through the lens metering. The camera sends out a preflash signal, determines how much light is reflected off the subject and sends the information to the strobe, which fires accordingly. Optical slave strobes generally try to mimic the burst put out by the cameras built in flash, so you have to cover it but leave it on. If you don't cover the built in flash, it is too close to the camera lens for most conditions and will be reflected back by bubbles and impurities in the water as "backscatter".
