What is the ruling on the prayer of someone who performed only ablution without ghusl after swimming, and what should he do about the Two Rak'ahs upon Entering the Mosque if the congregational prayer has started while he is in the first or second rak'ah of them?

Swimming does not necessitate ghusl (a full ritual bath). Therefore, if someone was in a state of ritual purity before swimming, they are not required to perform ghusl or wudu (ablution) for prayer afterwards. If their wudu becomes invalidated, then only wudu is required. As for the ruling on interrupting a supererogatory (nafila) prayer when the congregational prayer is about to begin, scholars have differed on this matter. The preferred opinion is to complete it lightly if you are in the second unit (rak'ah), and to interrupt it if you are in the first unit, based on the Prophet's (peace be upon him) saying: "Whoever catches one rak'ah of the prayer has caught the prayer."