After a light swim I climb out of the water and sit down at the poolside.
Literal
Light [adverbial-く] one-swim did I [topic-は] water [from-から] rise-[and-て] poolside [at-に] waist [object-を] set-down.
Narrative present tense, first-person. ひと~ is a productive prefix meaning 'one X, a single X, a quick X' — ひと泳ぎ ('a quick swim'), ひと休み ('a short rest'), ひと眠り ('a nap'). プールサイド is wasei-eigo. 腰を下ろす ('to sit down,' literally 'to lower one's waist') is the set phrase.