When estimating how much capacity you need, consider that in ordinary wet exits, you might find the cockpit only one-quarter to one-half full. If you're dealing with breaking waves and really messy conditions, it could be completely flooded. Check the manufacturers specs on the volume of your kayak's cockpit. Likely you'll have to take the total volume and subtract the volumes of the dry compartments. Use the rating for the pump and the volume you think is relevant and figure out how much time will be required to empty the cockpit. A few minutes is reasonable.
Remember that in use, these built-in pumps will require that the water removed is replaced with air. If you have a neoprene sprayskirt, you'll see it sucked down and difficult to remove. You'll have to pull up one edge at the coaming to allow air into the cockpit every so often while pumping.
Check Valve and Through-Hull Fitting
For all these pumps, you'll need a check valve and a through hull fitting. The through hull fitting is a flanged nozzle that lets the water out of the kayak. You'll have to drill a hole through the deck or hull to put it in. The check valve connects between the pump and the through hull fitting and prevents water from coming back into the kayak.
Several kayaks come with built-in hand powered pumps as optional equipment. These are usually operated by a lever and are mounted on the rear deck just behind the cockpit or on the foredeck just in front. These allow the paddler to pump with one hand. Better, but not perfect. While you can add such a hand-powered pump to a kayak, for the cost and effort, a foot powered or electric pump is a better investment.
There are a few foot-powered pumps on the market and a few other hand pumps that have conversion kits available. Sea Kayaker magazine had a good article on foot pumps in Issue 92, Feb. 2003 by Thomas Finn.
Foot pumps are usually mounted on the forward bulkhead inside the cockpit. Some are mounted on brackets just aft of the forward bulkhead. How you mount it depends on the kayak, your leg length and the bulkhead position.
In the event that you need to pump out the cockpit, you take one foot off its footpeg and use it to power the pump (and if you get tired, you switch feet). Obviously, this means that you can't operate a rudder? if you have sliding pedals. If you decide to install such a pump, replace the sliders with gas-pedal-type rudder pedals. The good news is that you have both hands free to use your paddle.
In order to install such a pump, you are looking at possibly modifying, strengthening or replacing your forward bulkhead or arranging to mount some kind of bracket. In addition, you'll have to drill a hole through the hull or deck and mount a through-hull fitting to dump the pumped water. Inside the kayak, you'll also be adding a hose and strum box to collect the water from the bilge. A strum box is a widget that sucks up the water and has little holes so that flotsam in your kayak doesn't clog your pump.
Unlike electric pumps, the only way you'll run out of power is if you wear out your leg muscles. Since you'll be pumping mostly with your calf muscles, that can happen.
Electric pumps seem to have become popular in Australia before spreading to the rest of the kayaking world. The parts required to make them are readily available, but unfortunately, there are no complete solutions that you can buy and install. Well, actually, there are a couple of self-contained battery powered pumps, but they are quite bulky and are not suitable for most kayaks.
Electric pumps are easy to find and relatively cheap - marine supply shops sell small electric bilge pumps for sail and power boats. They are intended to remove the bits of water that accumulate in the bilge over time. Some are automatic and have water level switches; others are simple and are controlled with a switch. Whale, Attwood and Rule are common brand names. There are others that are likely worth investigating.
The power for an electric pump comes from batteries. Since you will be pumping for only a few minutes at a time and the pumps are relatively efficient, you won't need a big battery. The choice of battery type is not critical, but you have to decide what you want to deal with. Almost all marine pumps are 12V nominal (13-15V actual). Whatever battery type you choose will have to be kept in a waterproof container.
I just switched my kayak pump from AGM to NiMH - that should tell you what my recommendation would be. BTW - either NiCd or NiMH should be used with a ''smart'' charger; dumb chargers ruin batteries. Since these batteries are 1.2V, you'll need a dozen in series to get 14.4V (close to the nominal 12V of a marine battery). I used 2300mAh (2.3 Ah) rated AA NiMH. With these, you can use any AA smart charger.
Also - batteries can deliver considerable current - make sure you use a fuse with the pump; the pump manufacturer should provide a recommended fuse capacity in the installation booklet.
Estimating battery capacity
Take the amperage draw of the pump. Try to get actual and not rated draw, since not all manufacturers are as honest as you'd like; these values are available in some online power boat magazine reviews. Find out what the maximum recommended draw is for the battery type you're interested in (this is available on the battery manufacturer's web site). For example:
Attwood 625 pump draws 0.9 amps under load for 1 foot head (reasonable for a kayak). Round it to 1A. The battery type allows a maximum safe draw of 0.5C, where C is the rated capacity of the battery. Hence, 1 = 0.5C or C = 2. Hence you need a 2Ah battery.
You'll need a waterproof container large enough to hold the batteries, a fuse and other things. You can use a Nalgene-type jar or a Pelican or similar box (there are lots of nice small ones now). If you want to use a microwave container, beware: they are made with polypropylene and almost no glue will stick - not Aquaseal, not 3M 5200. You could even use a little box in a drybag. As long as it fits the kayak and your budget - go for it. If you are worried about the waterproofness of the container, consider putting it in a dry compartment like the day hatch. This means drilling a hole through the bulkhead for the pump and switch wiring and waterproofing that.
You'll need a way of turning the pump on and off. Waterproof electrical switches are hard to find and expensive. You also have to run waterproof electrical wire. And seal all connections. You can put the switch in the watertight box (so the switch only needs to be waterproof on the outside), but then you have to mount the box so that you can get to the switch.
I decided to scrap the electrical switch after it failed and used hot-tub/spa widgets. You can get an air button and air switch at a spa/hot-tub/pool parts store. They connect with a vinyl air hose - like the ones used in aquariums but with 1/8 inch inside diameter. They are completely waterproof and safe for use in the bathroom. This puts the air switch into the same watertight box/container as the battery. The air button can be mounted through the deck so you just push to turn on - just like the whirlpool. Instead of running wiring for the switch, you run a thin air hose; the only wire will be the one to the pump itself.
Put the electrical parts in the watertight box. Drill the hull for the through-hull fitting and, if desired, the switch. Mount the pump in the bilge at a low spot. Test. Go out rolling and never get sore arms from pumping again.
For pictures and a rough description of my installation: http://www.greatlakeskayaker.ca/pump.html