Although the United States of America contains a vast area of land, almost all of its allies and trading partners are overseas, and those interests require a powerful navy to support them.
The US Navy possessed some of the largest and most modern battleships in the world at the outbreak of World War Two, and despite losses during the Japanese attack on Pearl Harbor, was able to maintain a powerful presence in the Pacific. However, in the vast reaches of that ocean the battleship was no longer the king of battle. It was fortunate for the Americans that the handful of aircraft carriers then in service with the US Navy escaped destruction; given later events it is doubtful that a pure battleship force could have defeated the Imperial Japanese Navy.
The aircraft carrier became the main US naval asset during the war in the Pacific, which was very much a conflict between the air assets of opposing fleets. US carrier forces were hard-pressed early on but as the industrial might of the US was brought to bear, new carriers and air groups for them were deployed in such numbers that the enemy simply could not match their strength.
US naval forces were primarily engaged in the Pacific, but some capital ships and larger numbers of destroyers were deployed to the Atlantic theatre where their primary opponents were German U-boats.