Hi Tweeterdom,
I have heard this theory many times. However, all of the raptors that I've dealt with can open their feet (i.e. let go) whenever they want to. Think of aerial prey transfers during the breeding season or aerial kleptoparasitism during winter for example. How many of you have watched eagles chasing each other all over the sky contesting a dead duck or some such thing? When hard-pressed, the burdened eagle will often drop the prey in mid-air.
I had the great honor of living with a crippled Golden Eagle for 16 years (and a Bald Eagle for 7) and can assure you that they could both open their feet up anytime they wanted. I suspect that all experienced raptor handlers (rehabbers, falconers, hawk banders, raptor biologists) would agree that this is true.
Regarding eagles being pulled underwater by salmon, I have heard many eyewitness accounts of this behavior as well, including several reports from other raptor people including professional biologists. Although I have not directly observed it, I do believe these reports are accurate in many cases.
I've thought about this over the years and suggest two possible scenarios that could be involved.
First, at the point of capture the back talon (or hallux) may be driven into the fish skull or other cartilage quite hard. An analogy would be like a nail driven deep into wood. The talon cannot be pulled out and if the fish is large enough, the eagle is then pulled underwater.
A second possibility is that a talon is thrust in between the ichthyic vertebrae. In response, the fish arches its back sideways and inadvertently clamps down on the claw like a vise. Then they both go under. I suspect that it would not take long to drown an eagle.

Bud Anderson
Falcon Research Group
Box 248
Bow, WA 98232
(360) 757-1911
bud at frg.org