a fleet
A large group of warships is called a fleet. Fleet typically consists of multiple types of ships including battleships, aircraft carriers, destroyers, and submarines, and is often organized by a navy for specific military operations.
A group of battleships built to the same design is called a 'class' and is uslayy named after the first ship constructed. For example, the Iowa Class contains four nearly identical battleships, the first of which was named the USS Iowa. A group of battleships operating together is a task force.
Battleships are no longer used in modern warfare. They have been replaced by the aircraft carrier support group.
An organization of military vessels belonging to a country and available for sea warfare. It can be a group of warships of several types. During the Russian-Japanese War in 1904, a primary "naval force" was based on battleships. During WW2, a primary "naval force" included aircraft carriers.
In terms of an absolute always, no. In fact, battleships no longer exist in active use as part of any world navy. Is it sound naval doctrine to escort aircraft carriers? Absolutely. Aircraft carriers are primarily engaged in strike warfare--long-range assaults on enemy formations and land targets. However, they face significant threats from enemy ships, submarines, and cruise missiles. Destroyers, cruisers, battleships, and other escorts form part of the protective 'shield' around fleet carriers, because with their planes carriers are basically floating targets, lightly armed and easily damaged. Prior to World War Two, it was assumed by most tacticians that battleships would figure significantly into a decisive "Mahanian" fleet engagement. However, engagements such as the Battle of Midway, the Battle of the Coral Sea, and the sinking of the Bismarck proved that Battleships were becoming obsolete as independent units. The age of the carrier had come. Battleships, then, were reassigned for Naval Gunfire Support and as carrier escorts, primarily in an antiair role. Destroyers have always provided a primarily escorting role. The name 'destroyer' is actually a shortened version of Torpedo Boat Destroyer, a ship designed specifically to counter fast, torpedo-armed combatants that attacked larger ships in packs. Destroyers have become uniquely adapted to hunting enemy submarines (a huge threat to carriers during and since WWII) and, in the modern age, defending against cruise missile/air attack. The modern Carrier Battle Group acts as both protection for the carrier and its invaluable aircraft and as added firepower in the strike role. Carriers are worth so much, in both literal cost and military importance, that they must be protected.
Squadron
Formation
Because a properly organised Carrier Air Group has more firepower than a battleship, and can reach further.
Carriers, battleships, and heavy cruisers. Heavy cruisers had 8" guns, light cruisers had 6" guns.
Squadron Leader.
Because they could take whole aircraft carriers. It's not that easy to shoot down a large group of planes flying right at you.
Any group of warships, under the command of an admiral, can be called a fleet. During the Battle of Tsushima in 1905, Admiral Togo only had 4 battleships in his battle-line; the Russian Admiral (Rozhestvensky) had about 8 battleships. They were both battleship fleets.