fulta -ful-flu- gismu

x1 (passive) floats on/in fluid (gas/liquid) x2; x1 is buoyant.

See also limna, bloti, sakli.

In notes:

bloti
x1 is a boat/ship/vessel [vehicle] for carrying x2, propelled by x3.
limna
x1 (agent) swims in fluid x2.
sakli
x1 slides/slips/glides on x2.
brabaktu
bar1=bak1 is a swimming pool filled with bak2, made of material bak3.
flulimna
l1=f1 (agent) swims on liquid l2=f2.
fluplu
x1 is a floating island of material/properties x2 floating on x3.