WWI-era submarines weren't good at diving in general, especially by our standards of submarining. The diesel engines they relied upon required oxygen as did their electric engine's batteries (as, of course did the crew) and the only way to fulfill either of those requirements was surfacing. Unless they were in the midst of combat or otherwise threatened, they generally travelled at the water surface like a normal boat. (Remember that scene in Raids of the Lost Ark where Indy clings to the side of a German submarine while it travels in the Aegean Sea?) In any case, the Germans did develop better electric submarines during WWII that were designed to stay submerged far longer than previously possible rather than sail primarily and submerge in reaction. It wasn't until the dawn of the nuclear-powered submarine, however, that it was capable of staying submerged for almost its entire voyage.
