How far can you see to the horizon on water?

A very rough or "ball park" figure is about 20 miles.

Comment: It depends on your height above the water. You could only see 20 miles if you were well above sea level.

If you are standing more or less at sea level, with your eyes just a few feet above the water level, you could see only about 3 or 4 miles. For example:

Eye level at 6 feet: horizon at about 3 miles.

Eye level at 24 feet: horizon at about 6 miles.

Eye level at 96 feet: horizon at about 12 miles.

Eye level at 270 feet : horizon at about 20 miles.