Watermelon and all fruits are not male or female. Fruits contain the seeds and seeds develop from eggs. Females have eggs while males have the smaller gamete called pollen for trees and sperm for animals.
Plants are different than animals. A plant is neither make or female. In some species have male and female on the same plant while others in the same structure like the same flower. (I'm not an expert but even I know this)
The difference they state is only the size of the fruit. The picking tip I guess should be get the more round smaller melon.
How to pick a perfect watermelon: tips from an experienced farmer

Who doesn’t love watermelon? Most people are obsessed with this delicious and refreshing fruit. Sometimes we think that unless you’re an experienced farmer, buying a juicy, tasty, fully ripe, and sweet watermelon is hit or miss. However, it doesn’t have to be if you look for a few things when you’re picking out your melon. In fact, you can tell whether a watermelon is ripe without actually cutting it.
Today, we at Bright Side would like to share some tips from experienced farmers that will help you to choose the finest watermelon ever.
Look for the field spot
The yellow spot, known as the field spot, is the place where the watermelon rested on the ground. Ripe watermelons always have creamy yellow or even orange-yellow spots, not white.
Look for ’webbing’
These weblike brown spots on the watermelon mean that bees touched the pollinating parts of the flower many times. The more pollination, the sweeter the fruit is.
’Boy’ and ’girl’ watermelons

Many people do not know that farmers differentiate watermelons by gender. For example, ’boys’ are bigger, have an elongated shape, and a watery taste. The ’girls’ have a rounded shape and are very sweet.
Pay attention to the size

It is better to choose neither the largest nor the smallest watermelon. Select an average-sized fruit. And note, please: large or small, the watermelon should feel heavy for its size.
Inspect the tail

A dried tail indicates that the watermelon is ripe. However, if the tail is green, it probably means that the watermelon was picked too soon and will not be ripe.
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