A "bridge of boats" is type of bridge which floats on water instead of having permanent pillars. It is built by linking boats and the first and last being anchored to the shores. It was used as a military technique since ancient times, being the fastest method for an army to construct a water crossing. This method was used in the 5th century BCE by the Persian Empire in attacks employing armies of several hundred thousand soldiers against the Greek States, to cross both the Bosporus and the Hellespont straits.
An current day example can be found in Willemstad, Netherlands Antilles.