I calculate that the surface is 2,23517E-8 feet lower at one foot away from a spot on the round 4000 mile radius Earth. At 1 mile, the surface drops 0.66 feet. At 10 miles, the surface drops 66 feet. Most large ships are not directly visible 15 miles out to sea.
Ratch
Simple proof albeit not practical for many. Seen that effect for more years than I like to remember.