there are a number of marinas in block. champlains rafts you off as many as 6 deep.
i prefer block island boat basin you get your own slip. many times you need to make a reservation months in advance
check out blockisland.com
the harbor can get very very crowded so
make sure you have good ground tackle and a dinghy if you want to anchor, since you could be anchored out in the back 40.
there is slow launch service.
i think the beauty of block island is that there isnt much to do. there are a few bars and good restaraunts, but not much else.
the island is a very natural non commercial place and is one of the nicest places you can stay in new england.
will you be going straight to block or down the sound?