'Future' does not mean you can just ignore simple physics. If you actually manage to create liquids which have less mass than air (which I cannot think a way of how it could work atm), I really doubt the physical procedures to make this happen would not be highly detrimental to normal marine lifeforms.It's a future, man. You can make breathable liquids so light it can float in air.
Not meaning to say this issue cannot be resolved. On the contrary I am quite interested in what kinds of solutions can be found for this. I just want those to be at least somewhat plausable. Things like this are perfect opportunities to make races more distinctive and interesting. The Liir are a perfect example with their ships having really high mass, but still being highly manouverable and not being affected by inertia normally because of the Flicker Drive (high frequency teleportation on extremely short range, for those who do not know it).