why shouldn't we drink water after eating a custard apple?!

Best Answer - Chosen by Voters

I really don't know why she tells you that.
I visit my grand parents sometimes and she has a custard apple tree in her garden
and we serve it together with a bunch of other fruit and there is always a jug of water on the table.
I really don't see any reason why it could be a problem. its most likely one of those myths, I've grown up with my parents telling me that I couldn't eat or drink certain foods with others because of bad effects. but later on when I got older I realized they were just simply myths which were passed on by their own parents.
just like how you believe what you read on the internet and hear on TV, people in the older times had people different sources of information. but today now that we live in the information age. it is easy to have access to information from acquired by scientists. but as technology advances the many things we believe to be facts will soon be proven as myths.

Your Mom probably just wanted to make sure you finished your custard apple and didn't fill up on water.