Real world test on specs The Sea & Sea YS-250 underwater strobe recently shipped and we discovered it's advertised specifications didn't match it's real world performance. We're still happy with the YS-250, but it encouraged us to put our other popular underwater strobes to the test.
We measured the guide number, dry weight, and fresh water buoyancy for each strobe. Results in the table below are intended as a comparison of performance in identical conditions.

Strobe Model	Advertised Guide Number	Tested Guide Number	Weight Air (lbs.)	Weight Fresh Water (lbs.)
Sea & Sea YS-250	32	22	3.75	0.25
Sea & Sea YS-350	32	22	4.13	(+) 0.56
Ikelite DS-200	24	20	4.19	0.06
Inon Z-240	24	18	1.44	0.13
Ikelite DS-125	20	14	2.81	0.25
Sea & Sea YS-110	22	14	1.5	(+) 0.13
Ikelite DS-51	17	14	1.38	0.19

Guide Numbers (Land, ISO 100, Meters) Weight (strobe head with batteries)
Notes on these results Guide number readings measured in the center of beam and rounded to the nearest 1/3 stop. All strobes flashed 3 times and each reading was within 0.1 stops. Only one strobe tested on each model.
Big is beautiful Professional level photographers want a bright strobe to fill flash scenes while aiming the camera into the sun. The brighter the ambient light, the brighter the flash required to fill shadows. Digital photographers looking to make underwater sunballs will frequently find themselves at an exposure of f22 which is like taking a photo through welding goggles. These high intensity, wide coverage strobes also tend to be bulky. If you want the big bang, you have to put up with the bulk.
Guide Numbers - not a great metric Guide number is a commonly used metric to compare the brightness of strobes and is described by this formula: Guide Number = F-Stop x Distance It's easy to determine a strobes true guide number with a flash meter, but the guide number will vary based on ISO settings and distance measured in feet or meters. Manufacturers listing are not standardized in feet or meters. To make it more confusing, some manufacturers list underwater guide numbers. A strobe will loose significant power underwater, but the value will change depending on visibility. So when using a guide number to compare strobes, make sure you're comparing apples to apples. We prefer to use land based guide numbers, at ISO 100, and distance measured in meters.
Further, keep in mind that most guide numbers are measured in the center of the strobe beam. Some strobes might be bright in the center, but fall off quickly at the edge.
Buoyancy Modern underwater camera housings are getting smaller and smaller. These low volume systems can be heavy underwater. Years ago most underwater strobes were positively buoyant and thus helped keep our rigs lighter and easier to handle underwater. As new strobes get smaller they have moved toward negative buoyancy. Currently only the Sea & Sea YS-110 and YS-350 are positively buoyant and the Ikelite DS-200 is essentially neutrally buoyant.
As underwater systems get heavier, more people are turning to specialized strobe arms such as the Ultralight Buoyancy Arms and the new 4th Generation Designs StiX arms with floats. We're currently measuring both of these systems for buoyancy.
We hope that market pressure will get manufacturers to design with buoyancy as a significant feature.