If the leak isn't obvious you need to increase the pressure in the boat to force air out of the leak. You can do this by taping a large plastic bag around the hatch rim of the suspect compartment. Then make a hole in the bag and use it as a bellows to increase the pressure in the compartment. Your boat may have a tiny hole drilled in the bulkhead to equalise pressure, you will then need to tape this up from the inside of the compartment you are testing.

Then, using a 50/50 washing up liquid-water solution and a paint brush, apply the solution to anywhere you think there might be a leak. When you see bubbles you have found the leak.

You can also fill the suspect compartment with water and see if you can spot where any drips are leaking from. This only works for some leaks where as the previous method works for most leaks.