I've seen lots of both blotched and diamondback water snakes over the course of 35 years of herping in Texas and never have seen one anywhere near seven feet. I think the books are about right concerning size ranges here. I have much less experience with cottonmouths but have not seen one that would challenge the size ranges in field guides.
:[snip]Can anyone say they've ever seen Cottonmouths or Watersnakes (any kind) as large as seven feet in this area? Larger? [snip]