The Navy Seabees came about shortly after the United States entered World War II. There was a need for construction forces to build advance bases who were miliatarized; not just civilian contractors hired by the military. The need for a militarized force arose because under international law, use of civilian labor in war zones was banned. So, for this reason, Rear Admiral Ben Moreel put in a request to the Bureau of Navigation to organize and man a Naval construction regiment. On January 5, 1942 Moreel was granted the authority to do just that. Construction workers were drafted (the first recruits being men who worked on Hoover Dam, and other major construction projects) and placed within one of the three Naval Construction Batallions (NCB) which formed the Naval Construction Regiment. The nick name 'Seabees' was coined from the latter two letters of NCB. The Seebeas were given their motto by Moreel himself, which was "Construimus, Batuimus"; meaning "We Build, We Fight."

The Seabees played a critical role during World War II in all theaters of battle. They built hundreds of advance bases, roads, bridges, etc. Their most famous being their participation during the D-Day invasion of Normandy on June 6, 1944. Seebeas worked alongside US Army engineers in demolition units to clear the beach of steel and concrete obstacles. Most were the first ones to come ashore; so they suffered heavy casualties from German fire. They heroically worked on, planting their explosive charges, which breached the defenses. The Seabees also put up pontoon causeways on the beach for troops and armor to travel over; making the landing more swift. Guess who also was manning the transport ferries which carried troops and equipment from the large ships to shore? That's right, Seabees. The Seabees contribution in the invasion of Normandy was a huge one; maybe impossible to do without them.

Since their formation, the Seabees have been a part of every major war and every minor conflict the US has been in. More recently they have been called upon for support in Bosnia, Albania, Kosovo, and southwest and southeast Asia. The Seabees not only perform in combat, but also aid areas which have been hit by natural disasters, and do humanitarian work world-wide.

Sources:

http://www.history.navy.mil/faqs/faq67-1.htm
http://www.seabee.navy.mil/welcome.htm