Plot hole: For the plot to go the way it was described, Newton had to wake up at the right time from his drunken stupor (that for some reason the real killer assumed would last eons), sail his boat, go somewhere at sea to dump the corpse, and then back exactly at his pier in the crowded spot where everyone knows him, and do a thorough bleaching of the cabin, all in less than half an hour, close to 15 minutes since he has then to go to Catherine's, which is half a mile away. So around 10 minutes being generous, for a hungover middle-aged man in bad shape and in shock and be there by 7:30, with the real murder happening around 7 (the wife says at 7:45 that it's been 45 minutes since they heard from him). Of course he also had to do that unnoticed, with no record or witness about a boat sailing in and out within a few minutes at dinner time, and likewise nobody saw the real killer running like mad at the docks - also of course, the whole stabbing was done without a single trace of blood getting on him.
Plot hole: Saint Marie is an island with an astonishing homicide rate, but even considering that, with an a missing person and a phone call reporting a stabbing at the boat of a notorious belligerent drunk, with the same person called by name as the culprit, it's quite inconceivable that in a relatively small town like Honore nobody looked for Newton Farrell and questioned him all night - especially being so easy to find, or even checked the boat. Instead, they are surprised by the bleaching of the deck the day after, proving they did not even look for a moment during the night.
Continuity mistake: Addison Patrick pulls out of his pocket a bag with the coins. A string dangles off his palm, but when he hands it to Florence, the strap has been pulled up. (00:30:20)
Revealing mistake: Dwayne supposedly swapped the rum with Humphrey, but if you pay attention during the time when JP hands them the glasses, it simply does not happen. (00:26:00)
Continuity mistake: Goodman calls it a day and mentions the stag party. Dwayne lays out on the desk three pair of handcuffs; the leftmost (the first he put down) is apart from the others, but when the lube comes into play, all three are clustered together. (00:22:35)
Continuity mistake: Florence puts on the table the knife sheath; it is laid out with the ends of the belt apart, but when Newton Farrell answers, they are joined together without anyone having touched it further. (00:20:30)
Continuity mistake: Humphrey found the cigar butt inside Newton's ship. He goes with his ever-smiling sidekick to interrogate him, finding him in asleep with a flask of booze between his hands. Cut, and Roger Griffiths' hands are on top of the flask instead than the bottom. (00:20:10)
Continuity mistake: Florence finds the divorce papers and calls Humphrey's attentions on them saying "Sir..." There is a paper sheet leaning by a corner on the laptop's right side. When the tabletop is again in frame (when Humphrey puts the coin back in the safe) the papers are arranged differently. (00:18:30)
Continuity mistake: The police are inside Tosh's boat, and Naomi Walker pokes downstairs to tell them that all his stuff is there and to help themselves. She has both hands on the manhole, which becomes just the right hand at the first cut. (00:17:25)
Continuity mistake: Humphrey grabs the ashtray in the boat; the cigar butts are placed differently on it between shots. (00:17:25)
Other mistake: In Naomi Walker's flashback when she gives Goodman her recounting of the evening, she gets the call from her husband when the phone is on the table - she got it with the phone in mid-air as she was standing already, instead. (00:11:10)
Continuity mistake: Sam Walker is holding back his dad Tosh, pulling him apart from the rival Newton Farrell. When Tosh snarls "Get away from my boat", his son has his left hand on his shoulder, but it's behind his back in the intermediate reverse shot. (00:01:25)
Answer: Most likely they had only tropical exotic drinks, he wanted an old fashioned English beer.