Yes, you are. Fortunately, Wiki backed my understanding of the situation: http://en.wikipedia.org/wiki/Enigma_Machine#Military_Enigma

In short, the Navy, specifically U boats, used a non-standard, stronger crypto (in this, more rotors) Enigma machine, called M4. They formed their own Enigma network called "Triton" which the allies called "Shark".

Due to their hardening, and less message sent for traffic analysis, these machines were "safer" against cryptanalysis.