So here's the actual lifeboat from the hijacking, currently on display at the National Navy UDT-SEAL Museum down at Fort Pierce.
Like
@Oldloser said, it's mostly enclosed. The movie has it like this: Phillips was able to tell the Navy which seat he was sitting in, so with a plan of the lifeboat they could figure a way to either take out the hijackers or give Phillips an opening to escape without harming the good Captain. USS Bainbridge is towing the lifeboat, they've got Muse (the leader) off for "negotiations", aka dragging his gullible ass to the brig under guard.
Now, the movie shows the SEALs trying their damnedest to line up shots on the three remaining bad guys in the lifeboat. "A is go. B is go. C is go.", kinda thing. The bad guys keep moving or going just out of sight, although they have no idea that they're being constantly targeted. One is in the lifeboat's pilot chair. He's easy. One is kinda to one side, usually visible. The third is actually behind that central support between the two forward-facing windows. So Bainbridge is towing the boat and the SEALs can only get a clean shot on two most of the time, because Bad Guy #3 is being jittery and not presenting himself the way they need. Now, the lifeboat's bobbing a bit, swaying a bit, but the Bainbridge is a destroyer. May be moving but it's not like the old days when a sailing ship's pitch and roll would mean the difference between a cannonball slamming into the magazine vs the foremast, plus they're aiming astern.
So the SEAL commander's like "If we shoot #1 and #2, #3 is gonna kill Phillips. He's already jumpy, threatened multiple times, holding a gun on the guy. We gotta take them all at once." Then he decides "Haul on the tow line". Which they do. It jerks the boat forward, Bad Guy #3 stumbles into view for a SPLIT SECOND, Commander goes "EXECUTE!" and they do one hell of a nice job taking all three with headshots.
I have little trouble believing they WERE headshots, because that's about all you're gonna see in a boat like that. The lifeboat set in the movie was just about a 1:1 recreation of the interior of the actual boat, with alterations made for the camera etc, not unlike how they did with Das Boot. It's cramped as hell in there even on camera. There's not room enough for chest or body shots not nohow and punching through the lifeboat with something stronger than a 7.62 would probably fuck up Phillips too.