Bobbing up and down might be uncomfortable but will not kill you. Hypothermia will, and the best protection against that is to avoid getting wet in the first place. So in a weightshift microlight, my number one choice would be to get a decent drysuit/survival suit plus a suitable life vest. Secondary items would be the PLB and something called a RescueStreamer. Or a smoke canister. Or something else that makes you very conspicuous, so the time to locate you is minimized.
As someone who lives on an island, where the only realistic route off it for me is 54nm across open water, this is a subject that I've give a lot of thought to.
Bobbing up and down might not kill you, but uncomfortableness isn't the problem here. The problem is conspiciousy. When warning a dry suit or just a life jacket, only your head sticks out of the water. If you've ever seen the view from a S&R aircraft (I've never been in one, but seen the photos/videos) you're realise that it's next to impossible to see a head popping out of the water when your looking at 10+nm of water in any given direction. The target is simply too small.
This was brought home to me at a S&R organised training evening when S&R showed us a photo from one of their missions. They told us that someone was in the water in the photo, wearing a lifejacket and asked if we could see them. We were in a nice warm room, with a photo that wasn't bouncing around, in good light and with about 30 people looking at it, and not one of us could see the head in the water.
A life raft is a much bigger target and much easier to be seen.
If you just use a dry suit, then you need something else to help you be seen. The smoke canisters (of which I have one) and rescue streemers are good, but also have some drawbacks. Firstly you need to hold onto them until needed (or have them attached to you in some way, which isn't always easy). If they aren't attached to you, then your chances of holding them until needed is slim, as your body gets colder even in a dry suit. They are one time use only, so once used, you don't have a second chance.
Interestingly the S&R team told us that most people let this off too early, when they first see the helicopter. Apparently you need to wait until the helicopter comes quite close to you or they are unlikely to be seen as they will be looking much closer to themselves.
PLB's are a good option, but as Peter said you'll need to hold them out of the water. Some of them (certainly the cheaper McMurdo one) doesn't float and hence the need to hold them out of the water. There was a test done of different models a few years ago to see how long it took them to report in real world conditions, floating on the surface of the sea (for the ones that floated). Most were between 1 and 4 hours if I remember correctly, and this is a long time to be in cold water even in a dry suit. The most popular one, the expensive McMurdo model NEVER got to report the position. It would seem to me that a PLB needs to be kept out of the water, and you just can't do this with your hands. It needs to be in a life raft to have a reasonable chance of working.
The other disadvantage of a dry suit, is that you need to have them for everyone on board. Ok if you're the pilot, but do you have dry suits to fit all your passengers?
And finally the bobbing about does have it's own dangers. Apparently spray from even a light chop in the water is similar to water boarding when you just can't breath without water spray going up your nose, which quickly leads to exhaustion. A spray hood is highly recommended apparently (though I don't have one).
That's not to say that dry suits are no good. They certainly keep you alive a lot longer, and are ok if you have one for everyone on board, but they do nothing to speed up the rescue, so you'll need every one of those additional minutes to give maximum time for rescuse.
It's also unrealistic to think that you'll be rescued inside an hour. This can be the case in ideal circumstances....you're talking to a controller (and many micro pilots prefer not to) you get a mayday out which is heard, and you get a good position report, the ASR helicopter is already in the air and not already on a mission, and the weather/visibility is good and it's not starting to get dark. I think in reality most rescues take longer, though I've no evidence to back this up....it's just the impression I got from our ASR presentation.
All this is not to say that life rafts are a panacea either. They suffer problems too. You need to get it out with you, you need to inflate it, and you need to be able to right it if it inflates upside down. Most importantly you need to be able to get into it. This sounds simple but having tried it in a nice warm swimming pool, I can tell you it's far from straight forward.
The one I tried with had an inflatable step and as a small/light person I didn't have much problems using the step. Most of the others took a few minutes to get in even using the steps, as the steps kept submerging under their weight. Looking UP at the life raft wall from the side if it, while swimming in the pool, I realised that without some aid to getting in (such as the steps) it would be impossible to get in from a swim, even in a nice warm pool.
It's worth thinking about if you plan to go over water. While life rafts might not be a perfect solution, if you manage to get into one you are in a LOT better suitation than if you are in the water in a dry suit.
Oh by the way. The S&R guys reminded people using dry suits that the insulation comes not from the dry suit but the clothing underneeth. Many people don't wear layers of warm clothing underneeth and hence the dry suit doesn't give them the time that they otherwise could expect.
Ditchings might not be very common, but there are is usually at least one every year in the UK.
dp