The Pentekonter was a Greek oared warship, patterned on a Phoenician style of vessel of the same period. (c.500 B.C.) It had 25 oars a side, hence the name signifying 50. It is mentioned by the historians of the era, including Thucydides and Herodotus. It was replaced by the larger Trireme in the fifth century B.C.
(An earlier vessel, the "Pentekor" dates from archaic times. It also had 50 oars and was described by Homer as a "large unsinkable ship" implying that it may have had watertight bulkheads.)
References: As above - Thucydides, Herodotus and Homer.