atom model was like a watermelon with electrons for seeds

An example of an analogy used to describe an historical model of the atom:

"Thus Rutherford's discovery shrank the originally widespread positive charge of Thomson's atomic model into a tiny atomic nucleus in the very centre of the atom, leaving the swarm of negative electrons on the outside, so that instead of being similar to a watermelon with electrons playing the role of seeds, the picture of the atom began to look more like a miniature solar system with an atomic nucleus for the sun, and electrons for planets…"

George Gamow (1961) One, Two, Three…Infinity. Facts and speculations of science, Revised Edition, Dover Publications, Inc., New York.

Read about analogy in science

Read examples of scientific analogies

Many examples of science analogies are listed in 'Creative comparisons: Making science familiar through language. An illustrative catalogue of figurative comparisons and analogies for science concepts'. Free Download.

Describing the electrons as being in a swarm is an example of metaphor.