There is a big difference between yachts and commercial vessels just so that you are aware. Yachts tend to have small crews that spend their time delivering the boat to different places, cleaning it, and hosting. Commercial vessels see a lot of days underway but it involves lots of maintenance.
Your best bet is to send a resume to the boats and then follow it up with a call after a week or two. There really is no substitute for being able to show up as well. Many captains will like to try a crewmember out during yard period and if they work out, they get to go sailing. Once you get a year of experience under your belt, it will get a lot easier to get jobs but the first one is hard. A lot of people get their start splitting between a galley/hosting type job and a deckhand job.
Finding out how to contact boats can be difficult. The commercial boats can all be found with a web search using terms like windjammer, schooner, barque, etc. For yachts, your best bet might be to walk down to the docks where they tie up and start asking around. Don't be shy.
I did the commercial sail thing for 9 summers and it was great work. You have to be willing to work quite hard but it is very rewarding.