I'm not sure how much info can be gleaned/reverse engineered from the reports about the sinkings of US subs, but this site (
http://www.csp.navy.mil/) has what seems to be some really good info about all of the US subs lost in WWII. I've just started playing around with stuff there, but it was interesting to learn that of the 25 US submarines lost through the end of 1943, 11 appear to have been lost to enemy surface ships and only 3 to enemy air attack (and one of those was while in port, though another 3 are attributed to mines but could possibly have been unreported air attacks), with 6 or 7 having been losts in accidents and 1 to an enemy sub. In terms of what sort of results to aim for in tweaking, it would seem that surface vessels should be about 2-4 times more dangerous than airplanes, at least during the first couple of years of the war.