Interesting. I've seen plenty of places where otters have used beaver dams or lodges to get in or out of the ice. I've followed their trails for miles, found my share of "toilets," and even manged to catch a few. I've also caught a few otter in under-ice beaver sets. I've never seen any evidence of otter targeting beaver, and frankly, the possibility never really occurred to me.
I managed to dig up one paper that has actual data (Reid, D. G., T. E. Code, A. C. H. Reid, and S. M. Herrero. 1994. Food habits of the
river otter in a boreal ecosystem. Can. J. Zool. 72:1306-1313.). They found 27 beaver-bits in 1191 turds. Another (unpublished and unreviewed) paper (Heidi Hansen, University of Wyoming, 2003) said "it is commonly contended by trappers in Alberta that otters are significant predators
of beavers," but offered no evidence. I also found in a USFWS bulletin written by a forester (or the technician thereof), and claiming no particular credence that I can identify, "Northern river otters may kill young beavers found alone in a lodge."
So, it looks like there are data to support otters eating at least some beaver, but 2% of a reasonable sample of otters having beaver in their turds doesn't evoke the words "favorite" or "major predators" to me. Otters are fairly frequent visitors to moose kills. I'd not be surprised to find moose hair in >2% of the otters in some areas I trap, but that's not great evidence that otters are killing moose either! There's also lots of acknowledged conjecture, which generally says nothing meaningful about actual behavior.
I'd think I'd have found an otter-killed beaver or two in all the time I've spent flying around following otters. I find plenty of fish-bits that they've pulled up on the ice, especially in the spring when they're spending more time on top. Assuming they are killing beaver, what are they doing with them?