As simultaneously as humanly possible, I shoot everythign I have at the Megalodon, power up the boat, and pray like there's no tomorrow. Which there probably isn't in my specific case.
Pray, specifically, for a second Megalodon somewhere really close by (like, coming up right under the boat). One with cannibalistic tendencies, preferably. Failing that, pray the bullets at least make Meggie slow down a second or three.
Once I've got the engine going, if at all, use whatever free arm I may have to hold the girlfriend because, frankly, we're probably still shark food whatever we do, and given the relative size of Meggie, Meggie's maw, and us, throwing either of us, even bleeding, in the water is going to do about nothing.
If for some reason I should feel this is workable at all, I may just shoot myself and jump in the water. But as outlined above, I find it unlikely that would pause the Megalodon more than a quarter-second.
So yeah. Not a brilliant plan for survival odds, but frankly, I don't think there's any good plan in this scenario.