Sea Riddle

[sudoku]

how about a cloud?

[qwer123]

a boat sail? the sail fills wiht air then thus moves the ship

[sudoku]

so what is the right answer? I typed cloud before because with "air" it "floats", but pouring "sand" into clouds will force gas particles to fall because of density

[Kalyra_Shadowdancer]

i'm with the person who said ballast things for submarines -- isn't that how subs go up and down? by filling tanks with air or sand or something like that?

[tifa_nicki_lockhart]

treasure chest? message in a bottle?

[koonkang]

the sea

[vavagrl1119]

the only thing I can think of is hot air balloon?