What should seas be filled with to make boats impossible without balloons

Boats and balloons both work because of buoyancy - they are pushed up by a force equal to the weight of displaced water/air respectively. I want my (otherwise earthlike) planet to have 'seas' of a liquid or gas that is

• Denser than air, so it sits on the ground
• Light enough that normal boats made of wood, steel, etc immediately sink (unless they were impractically large and light).
• Ideally semi-inert (at 1atm 15 C) and nontoxic (other than being a simple asphyxiant)

I plan to have balloon-boats - which are basically lighter-than-air craft (with hydrogen or helium balloons), except they aren't lighter than air - just 'lighter' (less dense) than whatever the seas are made out of.

What substance should I use?

(Bonus points for an explanation as to how a such a sea could form.)

posted about 6 years ago

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