After a captivatingly swampy silent rendition of "King Arthur" in 2010 -- and behind-the-scenes pledges to never do it again -- Synetic Theater is again transforming its Crystal City stage into a lagoon-like tableau, with standing water onstage and an audience "splash zone." This time, it's for Shakespeare's waterborne epic, "The Tempest," which begins Thursday. "It's just so cool," says the theater's technical director, Phil Charlwood. "It adds another world. We just go, 'Okay, we'll deal with the headaches.'"