Someone needs to go back to school and read up on the laws of conservation of energy, and the laws of thermodynamics.
Extracting hydrogen from seawater: 2 x H2O + energy(1) = 2 x H2 + O2
Burning the resulting hydrogen: 2 x H2 + O2 = 2 x H2O + energy(2)
Where energy(2) == energy(1).
In other words the energy that you use to create the hydrogen in the first place (by bombing it with radio waves, or electrolysis, or whatever) is exactly equal to the energy used when burning it (it's not the salt water that burns, but the hydrogen generated by bombing it with radio energy), HOWEVER heat dissipation and increase in entropy (read the 2nd law of thermodynamics) will ALWAYS mean that energy is lost in the process. So you can't use this on your boat to create free energy from the sea -- there's no such thing as "free" energy.
You always have to start with a higher energy (lower entropy) compound (such as diesel) and combust it in such a way as to create a lower energy (higher entropy) compound (such as CO2, water, smoke, fumes, etc) before you can gain any energy. Starting with water, extracting hydrogen, and then combusting the hydrogen to get water again, does not gain you any energy at all -- not in a "closed" system like a boat floating around in the water.
The only rational way to use hydrogen extracted from seawater to create energy is to (1) build a f**kin' huge power station, somewhere near the sea (2) use the power from that power station to electrically separate the hydrogen from the seawater, compress it and stick it in tanks and (3) ship those tanks to shore stations, boat owners, other countries, power companies, etc, who can then burn it to create energy. HOWEVER the energy used in (2) will always be more than the energy gained in (3). It makes a modicum of sense if the power stations are either massive pollution devices that nobody wants near their city (but can handle a clean hydrogen-burning generator nearby), or nuclear.
Find some desolate stretch of coastline somewhere, e.g. along the Western Australian or South Australian coast a long way from any people, build a big stack of nuclear power plants there, use those plants to extract hydrogen, and then ship that hydrogen around the world as a form of clean power. But that's another argument for another day.
And for those wondering about the three laws of thermodynamics, they are:
1. You can't win.
2. You can't break even.
3. You can't quit the game.