Yes, this is a effin' hard mission at first, but you start to get use to it later.
First off, I dunno which you're using (PS2 or PSP), if you're playing on the PSP version, then there's a chance you can get through the mission. Firstly, hold down L and your aim will become slower, meaning it should be alot more easier to attack those boats coming your way. When you hit around the Jetty, there will be some explosive barrels (The red barrels with the warning sign implemented on it), use that to your advantage as it would blast the enemies shooting at you, and it could damage their boat (Depends on were the boat is at).
I'd recommend using fine aim though, it gives you more easier chances to attack the boat since it's steady. I believe if you have the PS2 Version, the button would be L1.
I dunno if this helps, but I've passed this game loads of times, and it's usually thanks to the fine aiming feature in the game.