Gavin, it's usually better to wear a wetsuit when you're using a mask and snorkel. That way you can lie down in the area that you're sniping in, and have both arms free. It'd probably be OK (If the weather was hot enough) to just wear togs or something, but your knees probably wouldn't like it too much.
The only other way really is to wear waders or something and stick your face in the water. But you'd end up with a sore back, wouldn't be able to cover enough area (Wetsuit sniping is far easier) and if the water was warm enough to stick your face in it without freezing your ears off :) you may as well have a wetsuit on.
Cheers & Happy Hunting