you can't recharge them U/W

no, yes and kill you see the latest DAN summery, nmh will kill your buddy as
well.

rechargeable batteries like the others run out. FFS charge them defore your
dive and if they run out just enjoy the dive

Pay attention to how much they weigh.  Some rechargeable are noticeably
lighter, which will make your light more buoyant.

Lee

All kinds of batteries work fine in a dive light underwater...just as
well as they do on the surface, in fact...just as long as the light
doesn't leak, the batteries are not pressurized or submerged.

  Shortcircuiting either type might cause a fire or explosion.  There
are also other considerations, eg. NiMH are self-discharging at a rate
of about 2% per day IIRC, their nominal voltage is also 1.2V as opposed
to normal batteries which are about 1.5V.  Discharging them beyond a
certain point (about 0.95V/cell) is likely to cause permanent damage to
the cells, etc.  LiIon on the other hand need special charging circutry,
are a bit more volatile, and typically come in 3.0V or 3.6V cells, which
ofcourse isin't very comfortable for lights designed for 1.2V cells.

  Using NiMH/LiIon you will also need to consider discharge rates.
NiMH cells are usually rated for 0.2-0.5C discharge (unless you get
special high-discharge-rate cells).  LiIon should have some current
limiter installed in case of bulb shortcircuit.

  /m