Nantucket is a lot nicer than Marthas vineyard which being closer to the mainland is easier to get to by ferrys. Plus you won't run into emperor obama in nantucket...
The boat basin is pretty much the only game in town if you want to dock. That said, the best way to end nantucket is on a mooring with more privacy and better vies. Too many people walking the docks at the boats basin. The town has a Very good launch service...
Forgot the distances.., it s about 70nm from newprort to nantucket, probably about the same from block. MV is on the way, you can't really go straight from Nantucket to block anyway,
Instead of MV, consider a stop at Cuttyhunk, one night. Then run up the bay and cross at woodshole with a lunch stop at Hadley harbor, VERY VERY pretty place.
Yeah water is cold, can't really swim till lay July