"Vandmændene svømmer i vandet."
Translation:The jellyfish are swimming in the water.
Assuming you mean "The jellyfish is..." then it's because the Danish can only mean the plural. If it were singular then it would be "Vandmanden".
If you meant it the other way around, then it's because of the "are" in the English sentence which means it has to be plural.
So the singular in danish is "Vandmanden"?
And I look up and the plural form of jellyfish in english is jellyfish, this plus your explanation about the plural/singular form in danish explains things for me.