Yes, you need to stay on Water Island.

We want to stay at Concordia and think we will be perfectly happy without a car. The question is: how to get out there and back? I understand the bus stops at Concordia but stops at 7 pm. Is there a taxi that will take us from the ferry and if so, at what cost? Also, can we get a taxi at 5 am on a Sat. AM to get us from Conordia to the ferry? Keeping Water Island in mind as a close second---