Milford-based Waters Corp. said that its chairman and chief executive, Douglas A. Berthiaume, plans to retire within two years. Berthiaume, 64, became president and CEO of Waters in 1994, when the company separated from Millipore Corp. He became chairman in 1996. Waters said its board has appointed a succession planning committee that will help choose a replacement for Berthiaume. It plans to engage an executive search firm.