Piston Pressure
Streams generated from this method of pressurizing water often vary in strength since the stream is completely dependant on how fast the piston can be filled and compressed. Shot distances can vary quite dramatically and stream strength can vary during the duration of the shot. For larger pistons, a lot of arm force is often require to achieve decent ranges. This can lead to poorer control of the direction the nozzle is pointing, thus lower accuracy. These type of weapons are often inexpensive but are typically not recommended for use in large battles except, perhaps, as base defence.

Air Pressure
This is the method of pressurizing water which put Larami Ltd ahead of the competition back in the days of motorized water weapons. The nice thing with air pressure is that a water blaster can be pre-charged before battle and this stored energy will remain in the reservoir/firing chamber until a trigger is pulled. As it takes water longer to exit an opening than air, the air pressure inside does not drop too quickly allowing for decent shot times. The main problem with air-pressure based water weapons is that it requires a fair number of pumps to pressurize the firing chamber and that the air pressure drops as water levels drop, resulting in the often-seen end-trickle. The other problem is that if the out-take tube gains access to the pressurize air instead of being submerged in water, the blaster will fire out a mist shot which also quickly drains the firing chamber's pressure. One interesting thing to note is that pressurized gas over water resulting in the water's gas content to be increased. What this means is that the water being fired from the blaster's nozzle may sometimes froth simply due to the once-pressurized water being released into a lower pressure environment, allowing the trapped gas to escape. This can cause a loss of stream integrity and shortened range.

See:
Pressurized Reservoir Weapon Usage
Separate Firing Chamber Weapon Usage

Elastic/CPS Pressure
For the Super Soaker® CPS-based weapons, Larami Ltd. has chosen to use elastic rubber firing chambers to pressurize pumped water as opposed to using air pressure. As the firing chamber is filled with water, its rubber walls, wanting to return to the original shape, push on the water, pressurizing it. A simple pull of the trigger can release the pressurized water onto an intended target. Interestingly, the pressure exerted on the expanded rubber firing chamber remains fairly constant throughout a shot, resulting in a nice, even stream. If the firing chamber is emptied of residual air, the stream produced will be virtually free from gas and remain cohesive much longer. (This is part of the reason CPS streams fires farther than their XP and SS counterparts)

See:
Constant Pressure System Weapon Usage

Ball-Bearing System
Small, toy squirt pistols often use a mini-ball bearing-based valve system in their nozzle. When the firing chamber (a small pump area) is being filled with water, the ball bearing in the nozzle is sucked back against the backside of the nozzle to close the nozzle, allowing water from the reservoir to be drawn into the pump and not air from the nozzle side. At the same time, a ball-bearing valve on the reservoir side opens to allow water to flow from the reservoir into the firing chamber. When the firing chamber is squeezed to pressurize the water, the reservoir valve is closed and the nozzle valve is opened simply by the force of the water, allowing water to be pushed out the nozzle and not just back into the reservoir. This system is simple and works well, but is also prone to sealant problems after prolonger use. As well, this system does not work well for moving larger amounts of water or storing higher levels of pressurized water.

Spring-Valve System
This system works in the same way as the ball-bearing system except that spring-loaded valves replace the ball-bearing valves. This allows for larger amounts of water to be displaced, but pressure cannot be stored in the firing chamber.

Pinch-Trigger System
The first Super Soakers® employed this trigger-firing mechanism. The overall layout of the pump-reservoir system is similar to that of the ball-bearing or spring-valve system. However, an additional tube-pinch is placed before the nozzle instead of the valve. The soft tube before the nozzle is pinched closed using a spring-based device. This prevents pressurized water from escaping out the nozzle until either the trigger is pulled or the pressure within the firing chamber becomes higher than the strength of the spring. The system can be considered an analog device in that the amount that the nozzle is opened depends on how far the trigger is pulled.

Enclosed-Trigger-Valve System
CPS and SC Super Soakers® employ an enclosed-trigger valve system. This trigger/nozzle system is housed within a solid piece at the front of the blaster and works similarly to the pinch-trigger system. In this case, the pinch-trigger is replaced with a spring-valve which is opened upon pulling of the trigger. The end result is a sturdier trigger-valve assembly, but the trigger is still analog and the amount it is opened depends on how far the trigger is pulled.

Max-D Valve System
The Max-D Super Soakers® series employs an novel enclosed-trigger valve system. This trigger/nozzle system is housed within a solid piece at the front of the blaster and works similarly to the pinch-trigger system. In this case, however, the trigger must be pulled beyond a certain point before the stream is released. Once beyond the threshold, the nozzle valve snaps open, unleashing a stream of pressurized water with a full opening. This binary valve system actually gives the Max-D series an added range advantage over similarly sized soakers. Upon releasing the trigger, the nozzle valve snaps shut.