A "bridge of boats" is
type of bridge

Bridge

A bridge is a structure built to span physical obstacles such as a body of water, valley, or road, for the purpose of providing passage over the obstacle...

 which floats on water instead of having permanent pillars. It is built by linking boat

Boat

A boat is a watercraft of any size designed to float or plane, to provide passage across water. Usually this water will be inland or in protected coastal areas. However, boats such as the whaleboat were designed to be operated from a ship in an offshore environment. In naval terms, a boat is a...

s 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

Ancient Greece

Ancient Greece is a civilization belonging to a period of Greek history that lasted from the Archaic period of the 8th to 6th centuries BC to the end of antiquity. Immediately following this period was the beginning of the Early Middle Ages and the Byzantine era. Included in Ancient Greece is the...

, to cross both the Bosporus

Bosporus

The Bosphorus or Bosporus , also known as the Istanbul Strait , is a strait that forms part of the boundary between Europe and Asia. It is one of the Turkish Straits, along with the Dardanelles...

 and the Hellespont strait

Strait

A strait or straits is a narrow, typically navigable channel of water that connects two larger, navigable bodies of water. It most commonly refers to a channel of water that lies between two land masses, but it may also refer to a navigable channel through a body of water that is otherwise not...

s.

An current day example can be found in Willemstad, Netherlands Antilles

Willemstad, Netherlands Antilles

Willemstad is the capital city of Curaçao, an island in the southern Caribbean Sea that forms a constituent country of the Kingdom of the Netherlands. Formerly the capital of the Netherlands Antilles prior to its dissolution in 2010, it has an estimated population of 140,000. The historic centre of...

.