Complex sentence: Few actual galley battles in the provinces are found in records, but one action in 70 AD at the uncertain location of the "Island of the Batavians" during the Batavian Rebellion was noted, and featured a trireme as the Roman flagship.
Split into simpler sentences: Few actual galley battles in the provinces are found in records. One action in 70 AD at the unspecified location of the "Island of the Batavians" during the Batavian Rebellion was recorded, and included a trireme as the Roman flagship.