You will definitely want to hit Ballard's as the others mentioned, but having been to a bachelorette or two, I know you won't want to start the serious drinking until after 2 at the earliest... or the ferry and car rides back will be no fun at all.
I'd get to the ferry about 30 minutes before its scheduled departure. They don't let you on until 15 minutes before, as they are loading cars and such. But the earlier you are in line, the better, especially because the 10 of you will want to sit together and it will be hard to find seats together if you're running late. I've never been turned away for an overcrowded ferry, even on a Saturday, just seating will be your issue.
At the Ferry terminal in Block, there are restrooms and lockers, which will only fit a towel and small cooler (think kids lunch box). Personally, I wouldn't bother with bringing drinks. But if you have a rowdier crew, they may want them for the beach. But again, drinking in the hot sun all day takes it toll on bachelorette parties.
Directly across the street from the ferry landing is 'downtown'. Almost every shop and several restaurants are all right there, including bike/moped rentals and taxis. The beach is about a 10-15 minute walk from town, just ask anyone which way and they'll happily direct you.
If you'd like my very specific advice, here it is-
I'd get the 10:30 ferry, which will get you there at 11:30. You can grab lunch to go from any of the 10 restaurants/ pizza places/sandwich shoppes along the way, or have a bowl of chowder and mussels at the National Hotel on the deck, or a salad at Sharkey's. Head to Benson Town Beach, which is the closest one, but also the only one with lifeguards, snack bar, and SHOWERS. Have lunch, hang out, and shower around 3pm.
After the 10 minute walk back to town, hop in and out of shops, and grab some ice cream at Aldo's or The Ice Cream Shoppe, before you stop at the ferry landing to throw your bags and such into lockers. (You may want to hang on to towels for Ballard's so you can sit on the beach, or you can just eat at tables on their patio.) Then head straight to Ballard's (two minutes from the Ferry Landing). Have some happy hour drinks on the beach, listen to some live music, join a volleyball game... and if you love it, stay for dinner! Have a great time, you're so nice to be planning this for your friends!