The rechargeable have a slight lower voltage, but its sustain power is better. For camera flash, it recycles your flash much faster.
That's actually one of the best uses. Alkaline batteries suffer from depressed voltage under high current draw. That's what the "ultra" or "high power" alkalines are about. They don't store more energy, they're just able to deliver higher current for longer, making them better for high-current applications like camera flashes, RC vehicles, anything where the batteries could be depleted in a few hours or less.
NiCd and NiMH batteries are able to deliver higher current out of the box.
My hesitation has always been that if I leave the batteries loaded in the charger, with the charger powered up, eventually the charger will kill the batts. Sounds like this was due to the "dumb charger" I had.
Are the LaCrosse and Watson chargers "smart enough" that I could leave batteries in the charger, charger plugged in for weeks at a time, without fear of damage?
Smart chargers are programmed to recharge your battery, then shut off. They monitor the voltage to determine the battery's charge state, and charge appropriately (or don't charge if it would cause a dangerous condition). That said, all the "dumb" chargers I've seen recently also shut off. The light just stays on with a different color to indicate it's finished charging.
The main advantage of the smart charger I got (a MAHA C9000) is that you can vary the charge rate (a slower charge can usually "squeeze" more juice into the battery), it can completely discharge a battery, and it has two programs to condition the battery (charge, full discharge, then charge). The last one is handy for resuscitating batteries whose capacity has been reduced by poor chargers or just don't work as well because they're getting old.
I have found that if you live off the dock, rechargeable batteries are not much help, unless you have a 12 volt charger (like for my Ryobi cordless tools) or a solar charger. Many of the 110 volt chargers will not work well on the square wave inverters and most of us would rather not run our generators for hours on end to charge rechargeable batteries. I get very limited life from cordless tool batteries with 110 volt chargers.
Battery chargers use DC, not AC. They just come with an AC adapter plug since that's what's commonly found on land. My MAHA C9000 takes 12V DC input. So as long as your boat's battery bank was putting out close to 12V, it would work fine with direct DC.
I like NiMH batteries and use them a bunch, but they do have one quirk that makes them problematic: they are not 1.5VDC. Nope. Because of their chemistry, most only output 1.2VDC (Enloop is ~1.4V). Capacity is fine. Ampacity is great. They charge fast,, have low self-discharge rate, and will recharge many, many times.
But they won't work in all handheld devices. Two of my (inexpensive) digital cameras, for instance, read NiMH's 1.2V full-charge as a low battery condition & almost immediately shut down. OTOH, my Garmin 76CSx GPS has a menu setting for battery type: you can tell it you are using NiMH, and it will accept the lower voltage. And of course, simple gadgets like LED lanterns, FM radios, and so on don't really care.
Yeah, this is an important thing to keep in mind. I use Eneloops in my Logitech mouse. They last about a month (vs 3 months for alkaline), and when I pop them in my charger it says they're at 1.16-1.17 V and have about half their charge left - 980 mAh left vs about 1950 mAh full.
So clearly the mouse is expecting 1.5 V and shutting down just below 1.2 V, and the Eneloops are hitting 1.16 V when they're at about half capacity. For this particular application, 1 month is more than enough for me (I keep a couple alkaline AAs in my bag for mouse emergencies - just long enough to recharge the Eneloops so I've had the same set for 5 years).
But if you have devices which absolutely need 1.5 V, you can get NiZn rechargeables. Nickel-Zinc is a nominal 1.6 V chemistry, so they're a good substitute for devices requiring 1.5 V. They're only a bit more expensive than NiMH, though their capacity and endurance (lifetime recharge cycles) are lower. Just be sure to get a charger which can charge NiZn, and don't mix it up with your NiMH batteries and charger (improper charging can cause an explosion or fire).