Yeah, the idea is that all the water is coming from ice-rich planetesimals that would usually have been incorporated into gas giants. In most cases these worlds would have formed at the inner edge of the Outer Zone and then migrated inwards a bit, so they're very stunted gas giants - if they'd remained in the outer system then they most likely would have become GGs themselves.
Hmm, so when GGs migrate inwards and get hotter or the star gets hotter over its lifetime (ala Sol), can the innermost GGs lose their H/He atmosphere and become Panthalassics?
Is there anything to be concerned about with solar tides (I am aware deep water tides are usually quite small) perhaps more at crust level/heating effects or with off-axis tides and spin oblateness?
Can a Panthalassic ever be solar tide-locked? (looks like it would take too long with a large planet) That might be interesting.
There'd only be floating vegetation if something like that evolved there or was imported, and there's no solid surface at all (unless it happens to be cold enough at the poles for ice to form). And with the nearest solid surface about 80-100 km below the sea surface, you'd need a pretty long anchor
Yes, it would have to have some form of station keeping thrusters. I am leaning more to having a submerged base/city with multiple very large towers popping up above the waves, but which can all be totally submerged if it gets rough.
Considering what industries could form on the surface of such a world; Is there any industry that benefits from readily available high pressure sources? (put it on a cable and lower it in the ocean). Perhaps making icebergs to order (possibly a form of Pykrete http://en.wikipedia.org/wiki/Pykrete
using local vegetation) to create usable land? Atmosphere/water extraction products? Seafood (and perhaps tourism in Space opera settings).