What tense should the underlined verb be in?

Last January, thirty members of the Polar Bear Club swum in the frigid waters of Long Island Sound.]]> What is the correct past tense form of the underlined verb?

Last January, thirty members of the Polar Bear Club swum in the frigid waters of Long Island Sound.]]> Say the sentence.

Last January, thirty members of the Polar Bear Club swam in the frigid waters of Long Island Sound.]]>