There are a lot of options:
a 4 cell pack will generate 4.8v, and be lighter than a 6v pack. Ni-Cad is the more usual type, but there are plenty of Ni-Mh packs around in 4.8v, which are lighter than Ni-Cad.
if that's still too heavy, then Li-Po or Li-Ion are lighter still, but cost more for the charger.
Check out the Kong Li-po's and Sapherion Li-Ion selections, both have a 4.8v pack available and are very light indeed. Both of these are far safer than the original li-po packs that tended to go pop rather alarmingly if you messed up the charging, both use new designs and new technology, and you can't make them go bang.
As I said though, chargers are a little more expensive.
swapping the motors from normal "can" motors to Brushless out runners would also save significant weight for the same power, and these can come in incredibly small packages, some weigh less than 5 grams. However, the ESC sadly is more expensive for these.
If your fighting for weight, I would steer way from the marine modelling resources and have a look at the Indoor Flying and Park Flying sections of the areo modelling world. The entire product range in these spheres is totally centred on ultra-light weight kit.
Battery packs weighing 30 grams, esc's weighing 15 grams, motors weighing 5 grams etc etc.
Steve