I've heard that using a condom over a mic is a good cheap way to record underwater. My question is, to have it tightly wrapped over the mic or have an air pocket?
Firstly any mic you wrap in this way will only ever be omnidirectional. You can put a cardioid in a condom, but due to the way cardioids work, they will become omni. Its best to try and expel as much air as possible from the condom. if you have an air pocket it is likely that the microphone will move around inside the latex and you will simply end up recording the sound of the microphone moving around inside the condom. Also make sure it is firmly sealed around the cable.