Blood In the Water

You look through the binoculars at the vast ocean around you. At first you see nothing but blue water, but after looking left and right some more, you see the megalodon tear up another ship in the distance.

After it's done ripping that ship apart, it starts coming for you.

You look around anxiously for something to fight it. All you can find is a shotgun with 20 shells, and a rusty machete in the captain's locker. You are having second thoughts about whether you'll survive and you don't know you'll do.

What now?