The Nikonos SB-104 is simply the absolute best underwater strobe system you can buy. The SB-104 was engineered by Nikon for professionals without a price point in mind to solve the limitations of all other underwater strobes.
First, let's talk about power. With the exception of the now-discontinued Ikelite SubStrobe 400, the Nikonos SB-104 is the most powerful underwater strobe made. It's designed to light up those things that other weaker strobes struggle to illuminate. Nikon rated it at a guide number of 32 meters (104 feet) above water and 16 meters (53 feet) when used underwater at full power at 100 ISO/ASA (compare that to Nikon’s 36 foot UW rating for the SB-105). Ikelite's different test procedures came up with a 44 foot UW GN rating for the SB-104, seen in their chart here. The only brighter strobe weighs over a pound more!
Speed is impressive, being rated at 3 seconds to recycle from a full discharge with a fully-charged battery. Needless to say, it can recycle faster when using TTL exposures where the strobe doesn't fully discharge. Recycle times do slow as the battery is depleted (usually only after more than a roll worth of photos) but the strobe will normally continue charging and flashing long past the 120 rated flashes, although it will eventually take longer than 10 seconds to recycle. Needless to say, this is more than enough capacity for three dives in a day using TTL strobe control without changing batteries. Please see below about the overheating monitoring circuitry--you can't just pop off more than 30 flashes in a row with no breaks, or the circuitry may overheat. As soon as it's cooled off, you're back to shooting. The SH-104 quick charger will work anywhere in the world, and it charges one SN-104 in about 2 hours. It will charge two SN-104s at once if your diving schedule requires multiple batteries, although charging time does increase to 4 hours.
Next, coverage. The SB-104 is designed to cover a 15mm lens underwater without a diffuser! That really helps eliminate lighting drop-off around the edges of your photographs, and also compensate for a strobe that's not perfectly aligned on your subject.
Reliability was improved in all areas. Worried about flooding through the non-user-replaceable o-rings on your strobe controls? Not on the 104. Nikon eliminated the potential by using a magnetic switch and a design that eliminated shafts through the case for all controls! There's an o-ring for the battery and one for the sync cord that are user-replaceable, and one on the inside of the sync cord socket that is replaced in the annual service, and that's it!
The design was dramatically improved over other strobes. The SB-104 has a full 100-meter, or 328-foot, depth rating. Technical divers, take note! The inside of the SB-104 where the electronics are is inside a sealed compartment that's filled with inert nitrogen gas. The strobe automatically monitors itself for moisture, overheating, and proper electronic functioning. What's more, the strobe has blinking light alerts on the underside where you'll see them for moisture, overheating from rapid successive firing, and if either of the flash tubes doesn't fire!
Exposure control--near perfect! When used with the Nikonos RS, the SB-104 offers matrix-balanced fill flash capability. It also offers full through-the-lens flash control with the Nikonos V and housed cameras with compatible circuitry. Do you prefer manual flash calculations? There are built-in aperture and distance scale charts on the back.
The SB-104 is part of an entire system to provide light underwater. Also included are a more robust tray that is perfect for the Nikonos RS, but can also be used with the Nikonos V or IV-A. The standard arm has a handy quick-disconnect button that makes removing your strobe to hand-hold for that perfect angle a snap. The strobe holding joint also has a disconnect pin, making it easy to remove. You can also remove the joint that is used with the SB-104 and use a standard Nikonos joint on the arm to hold an SB-102, SB-103, or SB-105! This strobe uses a standard Nikonos sync cord.
To sum it up, if you're serious about your underwater photography, you've probably already discovered that lighting is even more critical underwater than above the water. Don't you think it's time you got the best solution made to provide that light while diving?