The hydrogen would certainly float - away - it would take a lot of holding down
Far less trouble to cable the power ashore and produce the hydrogen on shore, you could then feed it into the gas grid without compressing it.
Liquid Hydrogen has only 1\5 the energy density of Jet fuel, any engine can be run off hydrogen but in most cases space and weight as well as cost makes it unattractive. Maybe it could be useful for ships where space and weight are not a critical factor