Mercruiser still ideling after shut off?

[Kalani 938]

Hey guys I've got a question for you. I have an 06' Tahoe Q4 with a 4.3L Mercruiser 190hp. I purchased it last October and have only ran it about 5 times. I took it out for the 2nd time this season yesterday and it cranked right up after we unloaded it and it ran across the lake just fine. I then turned the key to shut it off and it kept idling kinda rough for about 10 seconds and then died. Well, when I tried to crank it again it would idle for about 5 seconds and then die. The idle was very low, almost like it wasn't getting enough fuel? After about 20 minutes of this I turned it over and while it was idling low I put it in gear and it took off and ran okay. I then adjusted the idle screw b/c it seemed to be idling lower than normal. Then I ran it for a while and it just seemed like it wasn't running right? Sounded a bit different and just felt different all around? Kind of hard to explain exactly what was different, but all you boat owners know what I mean when your boat just doesn't seem right. When loading it up I killed it and it took a good ten seconds again for it to stop idling and die? Anyone have any idea what this could be? I did put stabil in it over the winter.