This level is remarkably easy, play through it once on Hardened, or even below, to learn the flow. Now to beat it there's a small amount of timing.
First things first, the merchant ships, I'm not going to even talk about them, it's ridiculously easy, just focus on the PT boats rather than the big boats, and you'll be fine. Spray the merchant boats when there's no PT boats around. I've never died there, I don't know if you can. When you are on the Bow gun use the Right Bumber, it gives you a bigger gun, along with your turret. And that slaughters the enemy.
Second The Zero's. Only fight Zeros when NOTHING else is happening, I don't even believe they can kill you but fight them for something to do.
Third the PT boats after you land. Nowe we're talking about the real challenge (about the only challenge in this level). When you land you'll go to the right gun. Kill two PT boats to your far left right away. Now fight some Zero's but keep an eye out to the right, DO NOT GRAB A SURVIVOR! You will die if you do. To the right will be two more PT boats, kill them, then load the survivor if you want, two more PT Boats will come by, slaughter them and grab a second survivor.
Then if you grab the second survivor it'll run out of time and you'll switch to the left side. Look behind the boat in front of you for your first two PT boats, kill them, again DO NOT GRAB A SURVIVOR, start looking to the right for your second pair.. When you see them, blow them both up, Grab your survivor. I believe the last two will come from the left, Blow them up. grab the fourth survivor if you want at that point. Then you go up front and you shoot at some Japanese Zeros, run out of ammo, and you've beat the level.
So the pattern in the water is two boats from the left, two boats from the right, pick up survivors, two more boats, pick up the survivor, switch and repeat. With a little practice you could probably do this with no death and get that achievement too (and I was able to shoot out the spot lights on the merchant ship every time as well for a third achievement)