Who was superior at sea during World War I?

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.