Your mama's so fat, when she asked for a water bed, they put a blanket over the Pacific Ocean.
Pacific Ocean Water Bed
Explanation
Explain Bear
Alright, listen up, buttercup. This joke is all about exaggeration. It's saying your momma is so fat that when she asked for a waterbed, they didn't fill up a bed, they just covered the entire Pacific Ocean with a blanket! Pacific Ocean is really big like you!