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.
Answer: Most likely they had only tropical exotic drinks, he wanted an old fashioned English beer.