Silfra Fissure, Thingvellir National Park, Iceland
The Fissure lies between the Eurasian and the North American continental plates. The plates drift apart at a rate of about 2cm p.a. The water in the fissure is glacier water from the nearby Langjökull and is filtered through porous underground lava for 30-100 years on its path to the fissure. The national park in which the fissure sits has been declared a UNESCO World Heritage site. The other options in this question are all spectacular fresh water dives, but not between tectonic plates.