Watermelons can cross pollinate with some other melons but Watermelons cannot cross pollinate with cantaloupes.
Citrons and Watermelons can cross pollinate if planted next to each other but cantaloupes and watermelons cannot cross pollinate.
So you can plant watermelon and cantaloupes together next to each other without risk of cross pollinating because they cannot do so.
That goes for honeydews as well so you can also plant honeydews alongside watermelons or cantaloupes and they will not cross pollinate.