What exactly was the American WW2 smokebomb used in naval warfare?
The ocean is featureless, floating smoke bombs were used to mark a spot on a body of water so pilots could get back to the same spot. Smoke plumes can be seen from a long distance, they also let you gauge wind direction and speed. 

Smoke bombs were often used when attacking submarines, if a pilot spotted a periscope or caught a sub on the surface before it dove he could drop a smoke marker on the spot before coming around to drop a depth bomb on it. Smoke bombs were also used to mark people in the water, for instance downed pilots or sailors, so they could be rescued.