When you're on the boat, look at the end of the boat and you'll see a small island with a tree and a rusty small boat on it. Note its location for later. Also, make sure you have a Rocket Launcher and a decent amount of ammo for it.

After planting the C4 Bomb, run off the boat and swim to the island I noted earlier. When you get on the island, get behind the tree near the small boat. If you time it right, Crowe's helicopter will fly in with you behind the tree.

Now, you need to make sure that the tree is between you and him. The tree will absorb any bullets he fires at you. Now, take out the Rocket Launcher and hit Crowe when he's near you and trying to strafe past the tree to get a shot at you. Do this quick, or else Crowe will manage to get a few hits in. As soon as you shoot the rocket, reposition yourself so that the tree is between you and Crowe again.

Keep doing this until you see the helicopter retreat and have fire on it's blade. Congrats, you've beaten Crowe!

This isn't really much of a boss fight. Clear out the soldiers near Crowe, then treat him like you would any regular soldier. His SAW machine gun is more powerful, so be careful.

As soon as the fight starts, switch to your OICW and activate it's grenade launcher (hit X). Now, use the OICW grenade launcher and Hand Grenades to clear out the soldiers near Krieger. They're actually more dangerous than Krieger himself. Make sure you use the front of the desk as cover so that it's harder for the soldiers to get a lock on you and start shooting.

Krieger himself is easy. Find some cover and activate your night vision just in case he tries to cloak. Take out your automatic weapon of choice and shoot his crotch, which is his weak point.

The only thing you should be worried about is that Krieger can occasionally toss a Frag Grenade at you while you're shooting him. If you see a vague shape come from Krieger and head towards you while shooting him, run because it's a grenade!

Just keep shooting his crotch and watch out for grenades. He'll be down in no time. He's so easy that one or two P90 or OICW magazines can kill him with ease in Easy or Normal mode.