The vests are a good idea, a better idea (in my opinion) is a dry bag with a clip that you can clip to your belt if you're going down. Generally all you're going to have is what's on your person, so the vests are in theory a great idea. In practice - well, the vests are bulky, you might forget it at home, and if you go into the water, you and the vest are going to be wet. Plus, in a tiny dry bag (like one of these:
http://www.altrec.com/nrs/tuff-sack-dry-bag-small) you can put an MRE, a PLB, a change of ultralight socks and underwear, a few of those micro sized space blankets and much more without it being too heavy. The tiny dry bags go under the seat in the 206 and 207 series pretty well - just size the one you've got, and they can be clipped in under the seat, so they can be independent of the operator of the airplane. For water survival, check out StormySeas (
www.stormyseas.com/) these guys make really comfortable and utilitarian jackets, and if I had some spare coin around, I'd buy one. The vests they make aren't bad either, but the jackets are the best. They're warm, the block the wind, and if I go into the water, they'll be able to recover my body! (I'm only partly being facetious, if you go into cold water...well, you're probably done unless you're extremely lucky) Also, make sure every pilot has a multi-tool (I have a leatherman, I like it a lot, there are other brands, but this is all I've ever used) and that they keep that and a working lighter in their pocket on every trip - even if they're not smokers. If all else fails, and you're stranded with nothing else, it's a damn-sight easier to start a fire with a lighter and kindling than with two sticks and a lot of time - especially if you're cold, and the airplane may have fuel that'll work great to get a fire going on wet kindling.
Anymore, I generally don't carry anything beyond my leatherman, and a lighter, and I dress for the weather - however, this is because we have SpiderTrax GPS locating and we're generally flying IFR. If I go down, there are only three outcomes. One, I die on impact (typical CFIT "What's that Goat doing up here in these clouds, am I still on the airway - WAM!"). Two, I'm picked up in 20 minutes. Three, I'm never seen or heard from ever again (i.e. I go down in the water as the tide is going out and I drown). That said, I'll probably be packing my winter drybag here soon, because in the winter if I go down, I may need to care for my passengers, so I'll probably set that up here within the next month. A dry bag with a gun, an MRE, and a few other tidy things is a very useful tool.