absolutely sad, no it isn't.
From my perspective it's also eerie, dark, occult, and brutal.
I find In my Time of Need very happy, and I do not connect Watershed with sadness what-so-ever. nor with Harvest... or most Opeth songs.. they are not that sad at all..
or well.. the perfect time to listen through watershed is a rainy summer in the swedish countryside. and some people might find rainy summers sad.. but I love them, which makes it happy.
Katatonia has the mellow calming sadness to it that I love, but not Opeth.
and Britney Spears is just a sad human being..
^ If you ignore the lyrics, Burden is very happy in an awesome classic rock sense...