[url=https://onepiece.fandom.com/wiki/SBS_Volume_41#Chapter_398.2C_Page_206]I'm going by the author's own words here[/url], though with translations that can still be iffy. Luffy can still stretch because the Devil Fruit changed his body permanently, sea water can't make him not rubber or make Brooks not a skeleton, but it apparently prevents people with powers that need to be activated from doing so.