Vehicle control during hydroplaning requires special techniques that are different from normal driving on the road. The brakes should not be applied when hydroplaning; otherwise the vehicle may be imbalanced. Steering input should be smooth. Otherwise, control can be completely lost.
Hydroplaning occurs when there is too much water on the surface of the road for the tires to maintain a proper contact patch, causing a temporary loss of traction. When there isn't sufficient tread to wick the water away from the contact patch, surface tension of the water causes the water to actually lift the tires off the surface of the road. When the tires lose contact with the road, it doesn't take much force for the vehicle to enter a skid.
Movement inside the car, if violent enough, can actually shift the balance of the vehicle enough to cause a skid. Even a strong lateral wind hitting the side of the car at an angle can cause a hydroplaning skid.
Also, unless corrected, once a skid begins, it will worsen, until the vehicle enters an uncontrolled flat-spin. If the vehicle is going fast enough, there is enough unobstructed space on the roadway, and the skid is harsh enough, the spin can last for multiple rotations. However, not every hydroplaning skid causes rotations; the vehicle that is hydroplaning can turn sideways and continue traveling in the same direction of travel, but ninety degrees to the roadway. If you're extremely lucky while hydroplaning, your car won't even enter a skid. You'll just have the feeling of a loss of control, as well as feel as if someone is pulling your car backwards, or slowing it down severely.
The Impact of Tire Design on Hydroplaning
The tread of vehicle tires is specially designed to channel water
on the road surface to the sides of the tire, maintaining an optimal contact
patch 
during operation in a wet environment, thus maintaining traction and vehicle stability and control. The design and depth of tread, vehicle weight, tire pressure, and the water depth combine to determine whether a tire will hydroplane. Hydroplaning particularly occurs at higher vehicle speeds, because there is more air moving under the vehicle, pushing the vehicle up and lightening the vehicle on the tires. The larger the contact patch, the lower the probability of hydroplaning. Lightly loaded tires with worn tread present the maximum risk. Deep tread disperses water more easily.
Under-inflated tires are more vulnerable to hydroplaning, particularly with a heavier vehicle, because the tire tends to cup in this situation. Vehicles with trailers may be exposed to irregular hydroplaning due to the irregular weight distribution. An empty trailer will hydroplane before the prime mover. Unidirectional tread designs, that appear to crisscross, are normally used for increased hydroplaning resistance. The numerous tread grooves are shaped to increase the tire's ability to channel the water out from under the tire.
Road Designs to Increase Traction and Control
Road engineers help to combat hydroplaning in a number of ways. A raked appearance, with grooves in the roadway helps to move the water off the surface of the roadway. Whether the road surface is asphalt or concrete, it is never made perfectly smooth, creating pockets for the water to settle into, allowing the tire to maintain contact with the high spots.
Road crown, the phenomenon in which the center of the roadway, between the lanes of a two-way street, is higher than the right edge (in the U.S.), creates a slope that causes the water to run off to the sides. Multi-lane two-way roads will normally have two crowns, one for each direction, with the center of the center lane being higher than either shoulder.
Vehicle Response During Hydroplaning
An experienced driver will immediately feel when the car is
hydroplaning, because the car will feel like it's skating. The
driver behavior during 
hydroplaning will depend on the travel direction, and the wheels that have lost traction. If the car is moving in a straight line, it will start to feel a little loose. Feedback to the driver from the road surface will abruptly end. Minor attempts by the driver to control the car will not have much effect. If the drive wheels are hydroplaning, there can be an abrupt increase in engine speed as the tires start to spin.
If the hydroplaning occurs while turning, and the front tires lose traction, the vehicle will rapidly start to drift towards the outside of the corner; if traction is lost by the rear wheels, the car rear will tend to swing sideways, making a skidding movement. If all the wheels start hydroplaning simultaneously, the car may slide straight. When the traction is regained, there can be an impulsive jolt in the direction of wheels.
Vehicle Response during Hydroplaning
The most important thing to remember if your car begins to hydroplane is don't panic. Panicky reactions are jerky reactions that usually include input that is both excessive and abrupt. If the rear wheels start to slide out from under your car, GENTLY turn INTO the skid. What this means is if the passenger side starts to come around, gently and slowly turn the front wheels into a gentle right turn until control is regained, while GENTLY tapping the brake to slow you forward momentum.
If the front wheels lose traction, turn them towards the slide. In other words, if you're in a right turn and the vehicle starts to slide, or push, forward, gently and slowly turn them that way, so that they are spinning correctly, while quickly and gently pumping the brakes.
Under no circumstances should you pump the brake pedal hard enough to FEEL the car slowing; you'll only cause an increased loss of traction and case the skid to worsen. Small driver inputs, whether they be steering inputs or brake pedal actuations, are the key to regaining control of your car while hydroplaning.