PFDs are often pretty heavy. Alpacka have this one http://www.alpackaraft.com/product/astral-v-eight-pfd/ at 554 grams which is (I imagine) about as light as they get. I discovered that inflatable PFDs you buy from boating supplies shops have an airline PFD inside them. When I stripped one down it weighed 282 grams as shown and should be adequate for the job.
The Ultralight Hiker
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