The beach just south of this marker is where most launch on Shelter Island. No real parking issues here.
https://goo.gl/maps/Qy5jdHbU8e32
This is the marker for Kellog's Beach which is across form the Harbor Police Guest Dock. There are two hour parking limits here...not sure how often this area gets patrolled though.
https://goo.gl/maps/gYcWTP3ohjC2