Can it fly 100 ft above the ocean? Yes, without any difficulties.
Can it achieve a top speed, 100 ft above the ocean? No, because the closer it is to sea level the higher the atmospheric pressure is and therefore drag is much greater. That is why commercial jets climb at a cruising altitude between 30 to 38 thousand feet because the ambient pressure and density is less and more ideal for a smooth, minimal drag, & fuel efficient flight.
Now lets imagine that a 747 doesn't have any device that limits any inputs on the parameters that may result in a safety issue. It will now TRY (But will never do) to achieve a top speed at 100 ft above the ocean. The result? It will encounter immense structural stress on the aircraft because of high drag. High drag would also mean that the engines would need to pump up more thrust to gain more airspeed & ground speed - resulting in a very high fuel consumption.
If you wanna know how it would look like if a 747 tries to achieve a cruising speed at 100 ft above an ocean: if your inside the aircraft, there will be lots of noise, lots of vibration, & lots of turbulence (As if the aircraft is still taking-off from the runway). If your spotting the aircraft outside then you'll notice that it would be loud, the jet wake or trail will be huge the wings will be flexing a lot & etc. And this is on clear, sunny, windless weather.
I took Aerospace Engineering and Aviation is a big hobby of mine.