Cracking Enigma wasn't a single event; there were variations in Enigma machines and the procedures for their use over the war. The earliest and most consistent successes of Bletchley Park were against Air Force Enigma; Naval Enigma, thanks to a more complex encoding procedure (briefly outlined [in a related question] (https://www.reddit.com/r/AskHistorians/comments/45unnm/how_did_the_germans_make_sure_their_enigma/d00hwoy)) was first consistently broken in mid-1941, mostly thanks to a series of "pinches" (captured codebooks). If you look at the tonnage of shipping sunk (in e.g. [ULTRA: Some Thoughts on its Impact on the Second World War, Williamson Murray] (http://www.airpower.maxwell.af.mil/airchronicles/aureview/1984/jul-aug/murray.html)), it rises from November 1940 to May 1941, then falls for the next six months.

The entry of the US into the war resulted in the "Second Happy Time" for U-boat commanders, a period of great success against disorganised defences in the first half of 1942. A new model of the Enigma was also introduced for U-boats with four rotors, and Bletchley Park were unable to crack this until the end of 1942, again mostly thanks to "pinches". With large numbers of U-boats operating in the mid-Atlantic, shipping losses again increased. The climactic battles of 1943 saw increased numbers of Allied escorts including carriers and long-range aircraft with improved weapons and detection technology, assisted by Ultra where available, inflicting unsustainable losses on the U-boats.

In terms of the overall impact of Ultra, a [previous answer] (https://www.reddit.com/r/AskHistorians/comments/40ta48/how_crucial_was_the_cryptanalysis_of_the_enigma/cyxr9md) may be of some interest.

Yes! Cracking the enigma was one of the most important milestones for the allies, as they could tell what the Germans were planning on doing. However, here's the catch, the allies can't let the Germans know that they cracked the enigma. If the Allies somehow just managed to evade every tactical bombing mission done by the Germans, then the Germans would know the Enigma was cracked and replace it.

That explains why the amount of ships that was sunk didn't decrease. However, the reason why the could've increased could be attributed to many different things. More belligerents in the war, such as the Soviet Union and the United States would've definitely added to the count of loss ships. In addition, after war started, many of the Allies could've started mass producing ships, which can take about 2 years to make, so when 1941 comes around and the years after, newly built ships start rolling out.