When Frenchy takes Capt Morgan to the cellar to remove a bullet, Frenchy unlocks the cellar door and promptly relocks it after they enter the cellar. A few minutes later, Slim comes down the stairs in the cellar with the first aid kit and the hot water. Frenchy has been in the room with the wounded man all this time and couldn't have unlocked the cellar door.
The scene showing the patrol boat firing at Morgan's boat, the machine gun is not recoiling, the bolt is not moving nor is there any gun smoke. This type of MG the bolt would recoil after every fired cartridge and would cause the whole gun to have some recoil effect. The gunner was just standing there, he would have to move his fingers or thumbs to fire the gun.
When Harry calculates how much Mr. Johnson, his fishing customer, owes him, he says more than once that it is 16 days x $35/day, + $275 for the rod and reel he lost. That totals $835. When he has Johnson's wallet he says Johnson owed him $825 (more than once). Harry calculates the amount and says, "Now you got a little credit so that'll be 825". The difference between 835 and 825 is, presumably, "...a little credit".