I heard there is a 700 foot wall of ice around the edge...
^^ Kind of like that wall
Word on the street now is that the firmament is ultimately what holds the water. So even if we melted the ice and tunneled through whatever land may or may not be there the water would be contained.
God's creation is more durable and safe then we imagine or beLIEve. It's not a place you can sail off the edge or have "planetary collisions". You can't fall off the edge or get sucked into a black hole. You can't be killed by a supernova or a meteorite larger than earth or an alien invasion. You can be killed by a hailstorm, a divine flood, the Apocalypse, or burn eternally in hell though.
In addition to not being able to have water slip out of the sides, I would also conclude that God prevents us from tampering with the sun and moon. Putting patriotic paraphernalia and coke cans on the moon or planning "Let's blow up the moon/sun", is probably something physically impossible for humans to accomplish.
So in addition to the height of the dome and low heights of rockets being a reason we can't get an image of the flat earth, also add that there may or may not be an impenetrable barrier/force that prevents us from going that high. Could be what is known as the "Van Allen Radiation" belts, that would certainly keep humans from meddling with the fabric of reality. Or those "belts" could be the telemetry data of the firmament itself, IDK.
I think more is keeping satan from draining the pool than an ice wall or the edge being "off-limits" by "they", the people who are acting on his behalf.
Did you notice in the intro to game of thrones the map curves up? It appears based in the intro sequence showing the map that they live in a hollow earth where you live on the inside of a ball and the curvature is reversed and the sun and moon and stars are in the middle.
This was confirmed to just be an art direction for the intro and I think the creator confirmed it's a globe.