El Grifo: Iran has considerable form for using lethal force against US warships and merchant ships, even in supposed 'peacetime'. Given the recent publicity it has given to its capability for swarm attacks and its avowed intent to drive western powers from the Gulf, I leave you to assess a Commanding Officer's likely attitude towards small craft showing potentially hostile intent in the area.
I can assure you from experience that when a go-fast approaches your ship on a steady bearing at a closing speed of 60-70 mph after ignoring warnings by radio, loud hailer, flags, signal projector (BIG flashing searchlight) and ear-splitting siren, the difference between warning shots and shots on target becomes moot extremely quickly. With only a .50 cal machine gun available (as in the case of USNS Rappahannock) there is little difference between the range (or time) at which 'warning shots' can be aimed sufficiently accurately to be effective and the range at which shots need to act as 'physical stoppers'. Concern for the safety of the ship and people for whom you have been given responsibility is suddenly concentrated above all else.
If you have the luxury of a helicopter riding shotgun further away from the ship, then things might be different but not necessarily so, especially at night when it becomes even more difficult to distinguish friend (or neutral) from potentially deadly foe.
As ever, we will have to wait for the results of the investigation to know what really happened. However, there will always be those who find it so much easier to make judgements from the comfort of their keyboard at home, especially when blessed with hindsight.