How do Piers 79 and 88 fit into your plan?

The St. George route on NYC Ferry stops at Battery Park City BEFORE Pier 79, so there's no need to go to Pier 79. Although if your goal is sightseeing then you can certainly go up to Pier 79 and come back down to Battery Park City.

In addition to the NYC Ferry St. George Route, note that NY Waterway (yet another ferry service, mainly serving NJ-based commuters) has a couple of weekday afternoon departures from Pier 79 that stop at Battery Park City on their way to South Ambody. Pier 88 is a cruise terminal, not a ferry terminal, and it's roughly a mile roundtrip walk from Pier 79 to 88 and back. So not sure how that fits in, although maybe there will be a ship docked there on that day that you want to see or something like that?