Yeah, it's a bug. Mine looks weird as hell too, because I have some more opaque bits but most of the water texture's about as transparent as yours. I think it might have something to do with how transparent your water is. But it doesn't bother me enough to jack up the transparency on my water, because I spent like, eight years getting it how I liked it.
I am sure this is not a bug in minecraft. To prove I am not crazy here are some relevant screenshots.
What it looks like in the default texture pack:
This is my water texture.
@khooplah1