Pistols
Most water pistols are small and often cheap. These tiny water pistols have a small pump inside them, and are typically filled through a small hole on the top. These are often shaped like actual pistols, and have a single-action shot. The gun only fires when you pull the trigger, and only shoots a small spray of water. It has a short range, and a limited capacity, but is compact and easy to hide in a pocket.
Water Cannons
A water cannon or blaster is a single tube with a plunger, shaped like the rear handle. These look like shotguns, and have only a handful of shots per loading. When you push the plunger down, the water sprays out the front unloading the blaster in a single or a couple of motions. Reloading one of these is very simple, dip the tip in some water, and then pull the plunger out. Water will rush into the chamber.
Pump-action
The "Super-Soaker" is one of the more famous examples of a pump-action water weapon. The gun has a hand-activated air pump, typically on the front, which fills a small air tank. When you pull the trigger, the gun releases that air and some water into the barrel. This pump increases your range and accuracy, meaning that it takes a little more work, but can fire far further than even the most powerful water pistol. They are far less compact, and do require some actual work.
Backpack Tanks
Most squirt guns have an on-board water storage tank, something that allows you to bring your ammo with you onto dry land. One solution is to bring more water with you, but it can be really heavy. That's where a backpack tank comes in handy. Certain water guns include a large plastic tank you can strap onto your back, connected to the gun section with a tube. While this does give you a huge amount of liquid ammo, it will slow you down a little if you are over-loaded.