Is psychology a science?
It's obvious. There is no copyright on the word, so:
Some areas of knowledge to which the word "psychology" is attached, definitely belong to what is properly called "science"
Other areas to which the word "psychology" is attached, definitely do not.
Is it a coherent discipline?
Ah, a much more interesting question because experimental psychology covers a very wide area.
One end shades off into neurobiology, the other into sociology or even social anthropology.
For the biological psychologist, the individual dissolves into a system of complicated molecules. For the social psychologist, our actions and feelings are understood as the result of our roles as cells in the social organism.