Early 80's Mercury 115hp outboard stalling issues

I'll start by telling you I'm a first time boat owner and know little to nothing about boating or motors. My buddy and I purchased a 1988 21' aluminum Starcraft holiday with an '83 (So i was told) mercury 115hp motor. The seller was very engine savvy having worked for bmw motors. He showed us what work he had performed to bring it back up to specs and also put in a brand new aftermarket starter for us. After taking it out on its maiden voyage she performed well minus taking around 10 mins to get her to finally start. She ran well and really took off. Again, minus the initial startup, she kept on working flawless. Recently, we have still had the issue of getting her started but now she stalls multiple times while idling and when put in gear it again stalls. We actually got stuck out on the water and had to get coast guard to tow us back in. As i stated i do not know much about motors but doing tons of research i decided to replace the fuel line and filter thinking it was stalling because it wasn't getting enough gas. (Let me add the boat is still in the water). Ran some sea foam and carb cleaner to no avail. Im stumped as to where to go from here. We have to drive the boat a few miles to the launch and none of our buddies have their boats in the water to tow us so we need to get her up and running and perform all necessary tasks while in wet dock. Thanks for any and all help!

Open the idle mixture screws 1/2 turn OUT (CCW).

Jeff