Gidds, I'd never be one to call "floating around and looking at things", boring. Very relaxing, yes. Boring, far from it. Skindiving to me IS pure diving.
You don't really need an "official" diving knife per se'. The shears idea is a good one, but if you think about it, since you have no air supply with you while you're tangled, you want something NOW.
I'd personally stick with a sharp knife. To me, "wild slashing" of a knife would be more efficient than trying to get my big fingers into a pair of shears, then trying to locate where to start snipping; especially if my lungs are screaming for air. I like a fairly long, very sharp blade, such as a fillet knife. A fillet knife purchased from a sporting goods store usually comes with a scabbord that you can attach to your weight belt. Or you can strap it to your leg.
Another thing you should be aware of is a phenomenon called Shallow Water Blackout, or SWB, (google it). That's when you've been at depth for a couple minutes, then on your way back up, you don't feel like you've been down that long, but expansion of your lungs draws what little O2 you had left, right out of your brain, causing you to immediately black out and drown. Usually happens at about the last two or three feet of your ascent when rapid decompression occurs. You shouldn't have to worry about that too much, unless you're out there trying to see just how long you can stay down. Just don't push yourself that far unless you have buddies watching you that are totally aware of what you're doing, and know what to look for. When alone, please follow Spectrums advice and keep your dives conservative. (We's really miss ya.)