Oh that’s easy! It fell of the edge of the Flat Earth. 
Some fun with Fountains of the Deep math:
Raising one kilogram of water from the Earth’s mantle (~1000 kilometers deep) requires 9.8\times 10^6 Joules of energy.
4184 Joules are need to increase the temperate of 1 kilo (one liter) of water by 1 degree Celsius, which works out to enough heat to increase the temperature of that 1 kilo by 2342 degrees C. This ignores any friction, or energy needed to convert that water from mineralized form back to liquid water.
Next, multiply that by the amount of water needed for a Global Flood. That’s a lot, certainly more than the amount of water currently available on the Earth’s surface. What does this to to global average temperatures? Considering only water, and letting the surface water be at 0 C in equal amounts, that averages to (2342+0)/2 = 1171^\circ C. I generally just round it off to "at least 1000^\circ C". This ignores the temperature/mass of land and air, but I’m already underestimating the amount of water and ignoring friction, so I call that a wash. (To date, no YEC has attempted to improve on my calculation.)
TL;DR: Enough superheated water arrives at the Earth’s surface to raise the average surface temperature by at least ~1000^\circ C. That’s a lot, enough to boil away the oceans and incinerate anything organic. A “Fountains of the Deep” scenario is not survivable.
Therefore, it doesn’t matter where the water goes, because no one will be around to see it.