Normaly the result of a huge gangbang where the partisipants all cum in a bucket. This is sometimes followed by pouring it on someones head.
The alturnative meaning is after a party a poool of white mess is left on the floor/bed/sofa/bathroom floor.
LIAM:No! It was like a sea of white.
AL: What a night! But we have to get this place cleened up. I don't want my olds comeing home to a sea of white.