I quite like my FlashFlood (even after repairing that damn firing valve again!

). I would even go as far as saying it's one of the last decent blasters SuperSoaker made!
You mentioned it could do with more water. If you mean in the pressure chamber then you can modify it to have larger capacity (see further on). If you mean the reservoir (where you fill it initially) you can add a backpack reservoir (either homemade (see
here and
here) or a SuperSoaker branded
max-infusion backpack if your FlashFlood has a
max infusion cap. You can usually find them on eBay.).
As for the pump on the FlashFlood, it's the same with most decent blasters. A large pressure chamber with higher pressure capabilties, requires a smaller pump diameter (sounds counter-intuitive but a smaller diameter pump can reach higher pressures. PRESSURE=FORCE/AREA. Smaller diameter=smaller area=higher pressure.) but obviously more pumps. To make a blaster pressurise with less pumps but acheive the same pressure, the pump needs to be longer, making the blaster longer. See
SSCentral's section on pumps (in the physics section).
The benefit of the FlashFlood is you don't need to pump it until it's pressure chamber (where the water is pumped into to fire) is full. It is technically a constant pressure blaster, as it uses a natural rubber pressure chamber which expands when water is pumped into it, generating the pressure. You only need to pump it until it's full if you want to fire the 'FlashFlood' nozzle. With the normal nozzle, the range is pretty much the same no matter how much water is in the pressure chamber (except when it's empty

).
In terms of new blasters, you're best bet would be something like the water warriors Gorgon or Colossus. At the minute the 'best' new NERF SuperSoaker is the
Hydrocannon, which, with a bit of modification, can have both a standard nozzle and a flood nozzle, and reasonable performance. As stock, it's pretty pathetic.
If you don't mind going secondhand, you have some really amazing blasters to choose from, albeit more expensive. As with all 'best blaster' threads, it completely depends on what you want from a blaster, whether it be insane range but almost no shot time (
CPS2000), or multiple nozzle versatility in a smaller package (
FlashFlood,
ArcticBlast (which some prefer to the FlashFlood) or
Gorgon) or even a fully automativc dribble shooter (most current NERF SuperSoakers!
isoaker.com is probably the best place to look for blaster reviews and statistics. You can search by brand, or performance characteristics such as range or output.
If you don't find exactly what you want in a stock blaster, you could always build your own! Most of us here have experience with building or modding blasters (one way or another) and would be happy to help out with any part of the technical or design side.
The other option is to modify the FlashFlood. You could give it a larger pump, or
power mod the pressure chamber.
Details on homemades and mods can be found on the
WWn wiki or
SSCentral homemades or
mods pages.
The WWn wiki page on
elastic pressure homemades explains the concept of the elastic bladder system used in the FlashFlood. The
air pressure page explains the other option.