The two-masted schooner Bowdoin, the ship that took Admiral Donald B. MacMillan and his crew on their voyages to the Arctic, will be just about as close as it can get to its namesake Saturday when it docks at the Maine Maritime Museum in Bath as part of Mariner’s Day festivities. Now a training vessel at the Maine Maritime Academy in Castine, the Bowdoin will be open for tours all day, and admission is free.