How long is a mile by boat?

A mile by boat is the same length as a mile by car, foot or airplane: 5,280 feet. If you're asking how long it takes to go a mile by boat, that depends on the speed it is going.

Whoops - I think I misinterpreted your question, so I'm amending my answer. A mile by boat is stil 5,280 feet, but there's something else called a "nautical mile" which is used mostly by sea and air navigators who can't measure long distances just by laying a ruler against a map, because any flat map trying to portray the spherical earth distorts distances. A nautical mile is equal to one minute of arc of longitude at the equator (or one minute of arc of a meridian, one of the imaginary circules around the earth the goes through the North and South Poles). This is about 6,076 feet.