sumo match with a water break
In sumo, a match that is interrupted by a mizu-iri (water break) when the bout is too long and the wrestlers need to rest and rehydrate.
After the match with a water break, both wrestlers got back on the ring.
水入り refers to the water break itself, while 水入りの相撲 is the match that includes such a break.
Compound of 水入り (mizu-iri, 'water break') + の (possessive particle) + 相撲 (sumō, 'sumo'). The term is specific to sumo terminology.