A stone in a marsh or shallow water that can be stepped on in crossing WordNet 3.0 & Open English WordNet

Stepping stones or stepstones are sets of stones arranged to form an improvised causeway that allows a pedestrian to cross a natural watercourse such as a river; or a water feature in a garden where water is allowed to flow between stone steps. Wikipedia

Stones placed to allow pedestrians to cross a watercourse Wikipedia Disambiguation

A set of stones used for crossing water Wikidata

A stone that can be stepped on in crossing something, especially a marsh or creek. Wiktionary