Honestly there is not a whole lot boat related under ten dollars. But there are always stocking stuffer needs. So then you decide are you going to go fun like a drink koozie with sailboats on it.
Or are you going to go practical like white electrical tape
SailorBag Sailcloth lunchbag- Perfect for a few drinks and a snack. This is ideal for a few hour sail or for the sailor in your life to bring to lunch. $15
The Marlinspike Sailor - This book is how to do all the fancy knotwork. Some fun stuff to do over the winter to learn better line handling and things like splicing triple strand. $12.75
Mclube- Ok not the sexiest gift or the most exciting but honestly almost every boat owner needs an extra can of this stuff. It is the best boat lubricant and so useful. So for the practical sailor this comes in super handy.$16.99
Leatherman Skeletool - I keep mine clipped to my shorts while sailing. The perfect combo of pliers and knife without anything much extra weighing it down this is my favorite multi-tool. This year mine ( with frequent washing) has done everything from crack lobsters to open beers to fix halyards or cut a new batten to fit. $39.89
New Sailing gloves- Go look at their old ones and get a new pair because racers will chew through a pair in about a year and a spare pair is always a great thing. Gil ones $25.36
This Ships Flask- Come one this is an awesome way to bring some rum down to the boat. at $25.39 this is sure to warm the heart of your favorite sailor.