unda

Unda, undae, f., is first declension. In prose it means a wave; in verse it very often means nothing more than water, so undis in Virgil is usually "in the water" rather than "in the breakers". Read it as "wave" every time and the poetry turns choppy for no reason.

The verb family is worth having: undo, "surge", and the compounds abundo, "overflow", inundo, "flood", and redundo, "flow back" - which is how English gets abundant, inundate, redundant and undulate. One letter separates it from the adverb unde, "from where", and both are common enough that the eye slips. Fluctus is the near synonym, and leans towards the swell rather than the single breaker.