The backwater rifle boasts the highest damage-per-shot of all 10mm weapons in Fallout 3, while also possessing the second-highest damage-per-shot of all rifle-type weapons. (The highest being Lincoln's repeater)
It should be noted however, that despite this weapon's attacks and critical bonuses each being 5 damage lower than those of Lincoln's repeater (resulting in 10 less damage total on a critical hit), the backwater rifle's 5x crit-chance multiplier is capable of offering up to a 90% chance to critical for a character with a maxed-out base critical chance (10 Luck, the Finesse perk and the critical chance version of survival expert), where Lincoln's repeater, with its lower 2x multiplier would only offer a 36% chance to critical with the same stats.
In this light, the backwater rifle may be more powerful in practice for high-Luck characters, as it offers a base damage of 45 (assuming no bonuses) with a 90% chance to achieve a 90 damage critical hit, as opposed to Lincoln's repeater which causes a base damage of 50, but only has a 36% chance of achieving its 100 damage criticals. This also makes the backwater rifle an effective alternative against tougher enemies, against whom Sneak attack criticals may not guarantee a kill.
The backwater rifle at 100% condition can fire 518 shots, or approximately 52 magazines, before breaking.