And that's apart from the challenge of actually deploying the raft, and getting into it.
In a low wing aircraft you stand a reasonable chance of landing upright and having some time to deploy the raft, then enter the raft from the cockpit or from the wings. If you're (very) lucky you might not even have to get wet.
A weight-shift microlight fuselage has no buoyancy to speak of, and will probably submerge up to the wing almost instantaneously. The wing itself might offer some buoyancy, but by that time you are certainly wet, and probably struggling to get out of the aircraft yourself anyway. Taking the raft with you in those conditions is going to be an incredible challenge.
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.