'Swim' is PIE *swem- — its strong conjugation swim/swam/swum has been stable for over a thousand years.
To propel the body through water by movements of the limbs, fins, or tail; to float on the surface of a liquid.
From Old English 'swimman' meaning 'to swim, float, move through water,' from Proto-Germanic *swimmaną (to swim, float), from PIE root *swem- meaning 'to be in motion, to swim.' The word is remarkable for its stability — it has meant 'to move through water' without significant semantic drift for at least four thousand years. The PIE root may also be the source of 'swamp' (low ground that 'swims' with water
The verb 'swim' has preserved its strong verb conjugation (swim/swam/swum) almost unchanged since Old English 'swimman/swamm/swummon' — the same three vowels, i/a/u, that alternated a thousand years ago still alternate today, making it one of the best-preserved ablaut patterns in the language.