In Warlords Ganymede has been terraformed to a state like the artic circle with it's tropics being like Iceland (this is assuming that Ganymede would have tropics)
The rich and affluent (as well as vacationers) of Ganymede don't want to spend a small fortune just to vacation in the inner planets. The head of ATLAS corporation had a grand idea: dome over smaller islands and terraform their interiors into a (Earth) tropics enviroment.
Now knowing a little bit on how islands work I think you could have the dome anchored into the underwater bits (I don't know a fancy name for it). But I don't know what else would be needed to keep the dome from collapsing. For if anchoring the dome their is a bad idea.
The regulation equipment is hidden under ground to keep the scenic views intact. Some would be located on the surface or if possible built into the roof of the dome (or all three)
Think of this dome as a almost complete sphere (has to grab the undersea parts of the island)