Find the power draw. The more you can get, the better, up to 12 amps; more than that can trip a circuit.
What is the peak horsepower? Horsepower measures maximum power. It is not the greatest indicator of performance, but sometimes the only specification available. Look for 1.5 to 2 horsepower on small machines and at least 3.5 on larger ones.
Check the seal suction in inches of water lift. If you need to vacuum water from the bottom of your pond and it's deep, you want to be able to do so. Look for 50 inches or more.
Find the air flow in cubic feet per minute. This tells you how powerfully the motor draws in air and factors in restrictions such as filters and bags. This is the best comparison between similar machines. Anything less than 90 cfm is not very effective.
Begin to look at standard equipment. Find the hose diameter. It can vary from model to model. The manufacturer matches the size to the power and capacity. A longer hose is more convenient, but length will reduce suction. Also check where and how the hose connects to the tank. You want a screw-on or locking connection so it doesn't pull loose.
Noise is the next thing to determine. Wet/dry vacs make noise and lots of it. Decibel levels about 80 require ear protection and 85 decibels and up is typical of unmuffled units. Those with mufflers typically are 60 decibels. You can add on a muffler for about $10.00 that will quiet the vacuum to acceptable levels, but still wear ear protection.
If you are using the vac to catch exhaust from a power tool, choose one that comes with a high-efficiency particulate air, or HEPA filter. Wood dust is a carcinogenic. Get a wet/dry vac that traps particles smaller than one micron. Look for a vacuum that has a filter bag inside the tank, so you don't end up with a cloud of dust when you empty it.
If you have a choice between big and bigger, get the bigger one. If you are vacuuming water, however, remember that water weighs about 8 pounds per gallon, so take that into consideration if you are looking at a 20 gallon tank
Steel is more durable than plastic. Tall vacuums tip more easily. Try pulling the vacuum by the hose. If it tips over, the vacuum is too tall.
Get a vacuum with a drain on the side near the bottom. That will save you the trouble of trying to empty the tank.
Get a vacuum with the longest power cord you can find. Make sure it comes with four sturdy wheels and attachments that store onboard.