Sometimes, when a lot of UFOs appear at the same time and I don't have the resources (or nerve) to go on all missions, I simply destroy one of the UFOs for a quick buck.
If you destroy an UFO over water, it disappears immediately. That's why there is the option to "Tail until over land", where there will be a wreckage left for you to destroy via airstrike (for a little money) or engage with a ground team.
Do you get the same amount of money you would get for the airstrike if you shoot down the UFO over water? Or should you always wait to shoot it down over land, even if you know you will destroy it anyway?