Probably lets Bryan write the Lusitania note, so the latter obviously doesn't resign. I doubt if he has any ambitions to be his own SoS. Beyond that, hard to say. Being from Indiana, he'll probably be more isolationist than Wilson, and might not oppose the Gore-Maclemore resolution the way Wilson did. America certainly won't enter the war during his first term, so much hangs on whether Hughes can unseat him in 1916. Provided he sticks to a reasonably progressive line at home, my guess is he has a fair chance to survive.
Not sure whether he'd make the Peace moves Wilson did. All US diplomats were advising that Europe would be deaf to such a move. Wilson ignored them, but Marshall might not. So it is then a question of whether he takes any retaliatory measures for British blacklists, etc, and if so whether this causes the Germans to reject USW, or at least tacitly exempt American ships from it. They'd be incredibly stupid not to, but then they sometimes were.