#Launching from international waters.

In addition to the factors mentioned in other posts, there's an additional benefit from launching from the ocean: you can launch from international waters. This could be handy if you're launching a rocket that uses some form of material or process that is illegal or heavily regulated for civilian use in your home country. For instance, you could potentially use the legal shelter of international waters to launch an Orion-drive/nuclear pulse propulsion rocket, since the Nuclear Test Ban Treaty bans its signatory nations from allowing the civilian use of nuclear explosives (though in this specific case, you'd probably want a conventional, chemical first stage to get it free from the water before you activate the main engine).

Launching from international waters.

In addition to the factors mentioned in other posts, there's an additional benefit from launching from the ocean: you can launch from international waters. This could be handy if you're launching a rocket that uses some form of material or process that is illegal or heavily regulated for civilian use in your home country. For instance, you could potentially use the legal shelter of international waters to launch an Orion-drive/nuclear pulse propulsion rocket, since the Nuclear Test Ban Treaty bans its signatory nations from allowing the civilian use of nuclear explosives (though in this specific case, you'd probably want a conventional, chemical first stage to get it free from the water before you activate the main engine).