Q. If life jackets that are commonly found on boats contained shatnez, would one be permitted to wear it while out on the water?
A. It is very unlikely that life jackets would be made from or contain both natural wool and linen fibers woven, and twisted or even compressed together.
Nevertheless, Horav Shlomo Miller’s shlit’a opinion is that even if that was a possibility, since Remah (Shulchan Aruch Y.D. 301: 6), rules that shatnez is forbidden only when it is worn as a normal garment, to protect from the cold, rain or heat, but it is permitted when one does not have any of those intentions and is wearing them only as a try out or for just transporting them, a life jacket would also be allowed.
However, Poskim disagree when it is a certainty or close to it, that the garment does contain shatnez if it could still be worn under the above conditions. The Rov advises to be stringent.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a