Yes, they can be fitted with .50 and 40mm grenade launchers, but at 3,000 rounds per minute, the 7.62 mini gun will shred a speed boat quite nicely. The tracer stream also makes it easier to guide onto the target from a moving boat.
Yes, but with a max effective range of 2000 meters compared to 1000 for the 7.62 a 50 cal is the way to go. This even more important when dealing with suicide mission targets. Now having 7.62 ready if they get past you outer line is good but nothing compares to the stopping power and range of a 50 cal on a small boat.