Slice cucumbers into quarters vertically, using a knife remove the seeds and set aside. Then dice the cucumber and place into a bowl.
Chop up the watermelon into small pieces that are the same size as the cucumber, and place it into the bowl with the cucumber.
Dice onion and place in the bowl.
Add the chopped herbs to the bowl.
Mix the lime juice and oil together and pour over all: gently stir.