Have you considered using a hinge like a piano hinge on each edge. One side of the hinge could be mounted on each edge. When it’s time to join another Hexatoon, two pins each half the length of the hinge could be inserted from the ends of the hinge.
I’m surprised that no one has mentioned the largest sea mount, the Mid Atlantic Ridge. I think it meets all criteria, not near territorial waters, under 1 KM depth, calm waters (waves 0 – 5 feet when I checked. Candidates are : Vladimirskiy Rise, 10deg 47.5 min N, 44deg 46 min W, Min depth 537 m; Rybin Seamount, 31deg 47.2 minN, 12deg 49.4 min W, min 412 m; Researchers Ridge, 15deg 23 min N, 51 deg 05 min W, min depth 760 m; Konstantinov Ridge, 33 deg 35 min S, 31deg 10min W, 497 m; Yermolenko Seamount, 42deg 24.7 min S, 1 deg 33.3 W, min depth 248 m. ref http://www.iho.shom.fr/COMMITTEES/GEBCO/SCUFN/SCUFN17-Report.pdf. There are more but I get tired typing.
How high is your island going to be? While waves in a bay may be only a few feet, I suspect at sea they are frequently more than 20 feet. Interesting concept though, but you may want to increase the size of your hexagons.