Watermelons are a large melonlike fruit of a plant of the gourd family, with smooth green skin, red pulp, and watery juice.
They are eaten as a fruit and are renown for their high water content.
Watermelon rinds are also edible, but most people avoid eating them due to their unappealing flavor.
They are used for making pickles, sometimes eaten in the Southern US.
It is used less often as a vegetable, where the rind is stir-fried or stewed. (source)
So can hamsters eat watermelon rind?
Yes they can nibble on them, however, they provide no nutritional value for them and so aren’t the best food for them to eat.