gently
slowly
downstream
downwards/downward
upwards/upward
In the dream my feet leave the ground and I start to float upwards/upward.

about (esp. BrE), around, away, back, down, off, out, up

seem to
Her voice seemed to float gently on the water.

across
A few small clouds floated across the sky.

down
chunks of ice floating down the river

in
pieces of wood floating in the water

on
A few leaves floated on the surface of the water.

freely
The government decided to allow the peso to float freely.

allow sth to

Float is used with these nouns as the subject:
ball, boat, bubble, cloud, currency, dust, log, mist, raft, smell, swan, word

Float is used with these nouns as the object:
cheque, currency, share