We had centimetric radar on aircraft before the germans even started snorkeling.
DMS-1000 (10cm radar, forward looking only) was operational on liberators in march of 1942.
AN/APS-2 (10cm radar) was operational on american aircraft from feb 1943
AN/APS-15 (3cm radar) was operational on american aircraft from nov 1943
I don't have exact timeframes for the british, but I'm sure it's similar.
From Uboat.net:
The first boat to be fitted with the Schnorchel was U-58 which experimented with the equipment in the Baltic during the summer of 1943 but operational boats didn't start to use it until early 1944 and even as late as June 1944 only about half of the boats stationed in the French bases had Schnorchels fitted.
The only german radar detector that I can remember seeing references to being snorkel mounted naxos. Range wasn't good, less than the range of the radar it was detecting. It was *very* rare as well. I don't think I have ever actually seen a picture of it in operational use. Further, it probably would have been useless vs AN/APS-15, though still could have helped with detecting the older AN/APS-2 when encountered.
A good portion of the problem is that the germans didn't trust their radar warning receivers at all. This stems from a british POW that convinced them that we where homing in on metox signals in august 43. This offered a believable lie to the germans, since we where detecting them with AN/APS-2 that the germans couldn't detect. From their point of view, attacks where just coming out of nowhere and this finally explained why. From that point on, the germans where completely paranoid about their receivers. Later designs reduced emissions, which reduced detection ranges and the captains still didn't trust them so they where often just not used, even when equipped.
A good solid read on the detection/countermeasure war between germany and the western allies can be found here:
http://www.ibiblio.org/hyperwar/USN/...51/ASW-14.html