The film is about survival. Watson's survival instinct was stronger. Captain Harris is maintaining the pretense of being in control, doing his duty, and doing what is best for the group. But actually he has already descended into madness. Watson was still sane, and did the sane thing. Which was to refuse to go on a suicide mission.
It's true he might have died with the others anyway. But he felt that was his best chance for survival. Either to be spotted by a plane or fly the new plane out of there.

Would you court marshal Sgt Watson for disobeying an order?

Watson was an interestingly complex character, and I think it was a cheap shot on the screenplay's part to add things to make him a more obvious bad guy. Those elements aside, I am entirely sympathetic with him and would've told the Finch character to shove it.

I don't sympathize a lot with Watson. I understand why he does what he does, but I feel much more compassion to Harris, who, while doing "suicidal" things, was looking out for everyone.
Two things made me hate him. His rage and disgust when Harris is found after he returns to he wreck site, and his smarmy "He's DEAD, isn't he?" reaction when Towns returns. Yes, everyone likes to be proven right, but his GLEE at being proven right is what makes him despicable to me. Yes, Harris and Renaud ended up murdered. You don't have to gloat about it.
I love to love my Princess.

Actually I'm more sympathetic to Watson than most. Yes, the faked injury was a bit ridiculous, but then he showed bravery and stood up to the suicidal captain later. He couldn't contain his glee at the Captain's death, but he was also glad he wasn't dead WITH him.
For who would bear the whips and scorns of Hollywood (;-p)

Yup, I agree with everything you said.

Watson gets discussed a lot here. My own opinion is that Watson
is
a low-life. Not for refusing to go on the Captain's suicide marchI wouldn't have gone eitherbut for the cowardly way he got out of it: faking an injury instead of confronting Harris directly. And then when he sees that Harris has returned, he leaves him outside instead of helping him, obviously in hopes that he'll die. And finally (when the Captain is killed after trying for the second time to save the others), his obvious glee when he asks Towns: "he's dead, isn't he?" makes me kind of sick. No wonder Towns punches him.