Maybe I just haven't had the right relationships, but I can't fathom putting a friend or partner before my own mother. No matter how close I ever get to someone, my mom will always come first in my heart.

So, I'd save her and there wouldn't be much of a thought process leading up to the decision.
Even if my mom said she wanted me to save my boyfriend because we're still young and need to live a full life together; I'd never be able to leave her behind. I wouldn't be able to forgive myself, and the guilt would be such a huge burden and throw me into such a deep depression that my relationship would probably crumble because of it. I'd live a terrible, dreary life until my misery comes to an end when I decide to get a euthanization while sitting by the water where my mother drowned.