What to Wear

We have three weddings this year coming up and I would like to not have to buy three different dresses. The one wedding I have no clue what is appropriate to wear and the couple didn't really provide much guidance on their invitations or wedding website. The ceremony is at 2 pm at a church. Following the ceremony, everyone is supposed to walk 1 block to a dock to board a ferry at 4 pm. The ferry is taking us to Thomas Island where the reception is being held. Following the reception, there is going to be an after party, but the location is still to be determined. It says on their website that the reception venue is semi-outdoors. The wedding is Memorial Day weekend. I have also never been to Boston which is where the wedding is. The invitations are rather plain so they don't really hint at the type of affair this will be. Any help would be appreciated.