Plastic or metal medium with counter sunk hole. Possibly threaded, or maybe there's a nut and washer nearby. A small boat will have limited mechanicals where a screw like that would be appropriate, I like the "headboard" on the sail as Johnb suggested. It could also be part of a roller furling unit. Check your lifeline stanchion bases. Even though most units that have removeable posts use a "set" screw...nah... it's a flat head, that wouldn't make sense.
What's interesting about this mystery is that normally we get queries on what the part is..... it'll be some odd shaped specialty tool. Here we have something completely recognizable and common... but maybe out of place on a small sailboat... so it could be just a matter of finding a hole that needs a screw!!!!

(Perhaps this screw found its way on board with a passenger, or a cooler, or a tool box.....something like that.
Anyway, it's only a 15 foot boat, how much time is it going to take to just inspect everything.... less than an hour? So get your flashlight out, and do your inspection. I am very curious about what you find, we hope you don't leave us hanging on this, post your result when you find out.