Ice [like-のような] water [object-を] despite [then-ものともせずに] he [topic-は] river [to-へ] dove-in.
ものともせずに means 'without being daunted by, braving' — a formal/literary expression. 飛びこんだ is a compound verb meaning 'dove/jumped in.' The sentence contrasts the danger with the bold action.