On the surface of the sea, the British Royal Navy was superior in this war. The German navy had its greatest successes with U-boats.
Although Germany had a powerful surface navy, it did not do much in this war. For the most part, it was kept bottled up in its ports in the North Sea. The Battle of Jutland was a major battle that helped to keep this situation in place. There is disagreement as to who won the battle on a tactical level, but strategically, Britain won because the German navy did not break out of the North Sea.
Germany had much more success with submarine warfare. German submarines inflicted heavy damage on Allied shipping. England, in particular, depended greatly on supplies brought by ship and the German blockade had a major effect these supplies. However, the convoy method eventually allowed the Allies to limit these losses.
Overall, then, the Allies won the war at sea. British surface ships won superiority and the British and Americans managed to protect themselves against submarine warfare well enough that German submarines could not win the war by themselves.