Best definition
Similar to Spitroast, Watertight is essentially the same, but with one woman, and three men. I.e. the orifices mouth, vagina

and anus are filled.

Boy, we really made Melody watertight.
Watertight: define #2
water tight
Expession used to describe a female involved in 4-way sex with three men. One man orally, one man anally, one man vaginally. This would fill all of the availible popular hole, thus making her “water tight”.
Last night we had that bitch water tight!
Watertight: define #3
Someone who does not leak information.
“It’s OK, you can tell me, I’m watertight.”