I think that there are three general contributing factors--and please understand that I understand the gross generalizations I am about to make---but years of trying to balance the force based on mission areas vice capabilites* required have not been able to answer the questions posed (*if half your missions are in the middle east, 20 percent are in Europe and 30% in SA--shouldn't your force look a little like that).
But the I can't swim is a real factor-just look at the sport of swimming--and that is were most of the Non-caucasians washed out--either actual swimming or hypothermia do to low body fat
-entry criteria--the ASVAB scores required to attend BUDS are fairly high, but they suffer the same flaws as other standardized testing--resulting in squeued results
--reason for joining the military--A far greater percentage of noncaucasians join the military specifically for the benefits and education and career training.
those three things contribute greatly to the problem. Please not, my first leading petty officer in 182 was an African American and I respected him and his leadership abilities immensely. I swam competively since I was 9. I worked with him everyday on his swimming, but he had zero body fat and weighed about 190--sunk like a stone and wound up not making it past the swim test. It was a true shame