Where to begin?
Get a copy of the Eldridge Tide & Pilot Manual and study the current charts and tide tables.
Tide causes the water level to go up and down. Current is water moving one way or another (SE or NW in LI Sound). They are related but not the same thing. The current runs one direction for around 6 hours and then the other way for about the same. High tide is about 1 hour later at any given point each successive day. Amazon.com: Eldridge Tide and Pilot Book 2011 (9781883465179): Eldridge Tide & Pilot Book: Books
Are you planning your escape from Riker's Island? (just kidding)
JM, the short answer is that the tidal current floods the Sound from both ends. West of Hempstead Hbr the rising tide comes from the East River, flowing east. The rest of the Sound is filled from the other end. It is possible to have a favorable current much of the way, depending on the state of tides and your boat speed. My most memorable cruises have found foul tides most of the way!