Taxi would be extremely expensive from JFK all the way out to Sayville so take the LIRR! Take Airtrain from your terminal to Jamaica Station and take the LIRR from there to Sayville. At Sayville station you can take a local taxi to the ferry. Airtain fare is $5, LIRR fare is $11.50 peak and $8.25 off-peak. Here's the link for the LIRR Sayville schedule. http://mta.info/lirr/html/ttn/sayville.htm
Taxi would be extremely