Your best bet is to contact some of the yacht clubs (Western LI YRA) and concentrate on City Island or Nassau County north shore. Find out when they have "beer can races" usually weeknights hosted by rotating clubs.
Show up on the dock, properly dressed, and start asking around two hours before race start time. SOMEONE ALWAYS NEEDS CREW. I saw one guy show up for a three-day race (the ALIR) wearning a sandwichboard that said "CREW NEEDS RIDE" on the front, and listing his resume on the back, and within a couple of hours--he had a ride for one of the biggest races on LI.
Sometimes it can be that easy. These days, many of the clubs can be found on the web--and some have "crew needed" pages that you can reply to as well.