True enough that Cottonmouth's (save for the very young) tend to be quite thick and do not appear to have 'missed any meals', but that does not exclude the possibility that ANY snake might be sick (for many reasons) or have suffered an injury making it difficult/impossible for it to eat. In any case....we shouldn't use body profile alone to identify the species.
There are similarities between the Cottonmouth and Diamondback Water Snake that could make identification difficult under certain circumstances, but the one feature that tells me this is a Cottonmouth is owing to the position the snake is in.
When looking over the top of a Cottonmouth (pretty much directly over it) you can not see it's eyes. Conversely, with the Diamondback Water Snake (and all other Water Snakes I can think of....) you can. The D/B water snake when feeling threatened will often times flatten its head (making it remarkably more triangular shaped) but even so...you can always see it's eyes to some degree.
I can see because of the condition of the snake the OP posted where you might be persuaded it was some type of Water Snake...but I think if you will spend some time looking at different pictures of both Snakes (especially from that angle) it might change your mind.
Nothing wrong with standing firm on an opinion IF you are fully convinced you are right. But do be 'accepting' of other's positions and their reasoning for it.
I am pretty well acquainted with both species and it was a tough call for me at first glance.
Diamondback W/S from roughly the same angle below: