The biggest cruise ship ever built in Germany was towed along a narrow river on Sunday en route to tests in the North Sea, DPA reported. The 315-metre-long Celebrity Solstice left the Meyer shipyard in the northern inland port of Papenburg after barriers were closed to increase the depth of the River Ems. The vessel, which cost 616 million euros (911 million dollars), has a 1,000-seat theatre, a night club and 1,400 cabins capable of accommodating 2,900 passengers. Workmen put the finishing touches to the interior decorations as the vessel inched its way along the river watched by thousands of onlookers, amid protests by environmentalists. The World Wide Fund for Nature (WWF) said that closing one of the barriers along the river to increase its depth had drastically reduced the amount of oxygen in the water. "The river is practically dead over a length of 30 kilometres," said Beatrice Claus, a spokeswoman for the WWF's German chapter. After undergoing tests in the Dutch port of Eemshaven, the floating hotel is due to go into service in the Caribbean for the US shipowner Celebrity Cruises.