There's no problem checking any watermaker parts
through TSA on checked baggage. I transported a complete Spectra 380c including the membrane and membrane tube three months ago from Boston, through England
and then onto Olbia. The liquid filled container isn't exactly a liquid filled container. Both times we transported new membranes to Europe
, they were shipped in a long plastic bag with only the membrane saturated with the liquid solution. TSA opened the cardboard outer box, and allowed the inner plastic bag containing the membrane to go through. Might not be a bad idea to provide a written product description inside the box so the inspector knows what it is.