Because he's the one that told Mama that the strawhats have the wedding cake Mama then replied if he's lying mama will skin him or eat him.
Which means if the cake is "drowned" it's better for him cuz she'll nvr find out he lied?
That's what I'm thinking. But then again Sanji's group is making a cake so if that turns up with the strawhats drown mama might just eat him either way for lying.
either way I'm sure he's worry is about how to save himself
Why is perospero so concerned about what will happen to the nonexistent cake..?