From: California, United States
I kind of thought of the floating fortresses almost a giant floating metal forts with large naval guns and maybe some missiles.
Based on the assumed background to 1984, things really went bad with WWIII occuring sometime in the 50s, with nuclear weapons being used. As a result of this war, the 3 superstates were born through consolidation and the various revolutions that gave birth to the 3 very similar ideologies that rule the world (Ingsoc, Neo-Bolsivikism, and Obliteration of the Self)
While all of these societies do focus alot of resources on warfare, with most of the technological advancements being limited to such pursuits, I think that as a whole, technology wouldn't have been as far along as in the real year of 1984, with warfare technology being closer to that of the late 50s/early 60s, with alot of persistent tech from the 1940s.
I think this is the case because the nature of the ultra-totaltiarian states would suffocate any free thinking on new ideas, which would massively slowdown most actual technological process. For example, the 'Rocket-Bombs' that are referred to in the book, I think, are probably not that much more advanced than the V-2 Rockets of WWII, except in accuracy.
So I envision the 'Floating Fortresses' as almost comic book style mass steel islands bristling with naval artillery and Rocket bomb batteries, that are so massive and armored that only nuclear weapons can have a hope of sinking them, which it seems clear that none of the superstates are willing to deploy nukes.