The Sphinx and the Pyramids of Giza were all built by 2500 BCE, before the Bible says Noah’s flood occurred.
According to the Bible in the year 2348 BCE the earth was plunged into a global flood which killed everyone everywhere because everyone was doing everything naughty (except Noah and his immediate family).
How do we know the Bible says 2348 BCE for the flood? Because in the 17th century an Irish fellow named James Ussher figured it out by carefully adding up the ages of the ancestors of Jesus as listed in the Bible all the way back to Adam. He also determined that according to the Bible the universe was created in 4004 BCE.
But hold on….among other historical problems with these dates is the fact that Egypt has been continuously occupied from 5400 BCE through today.
That not only means there were people living continuously in Egypt before, during, and after the flood was supposed to have happened and killed everyone according to the Bible, but that there were people living in Egypt even before the Bible says the Earth and the Universe were created in 4004 BCE.
How do we know how long people were living there? Through archeological finds, carbon dating and Egyptian record keeping we know that people were continuously living in Egypt back to at least the 5400 BCE date.
So… no, the pyramids have never been underwater.