Waves and tides as a source of energy wouldn't be efficient, but a constant ocean current such as the gulf stream would be. I have heard of plans (in legislation) dealing with sticking turbines in the current and enjoying efficient energy from it.
Yeah you might be out of power longer than a day. Generators are common and if you wait until after hurricane season to buy in anticipation of next year, there are huge discounts to be had. I should start an E-bay generator buisness!