You can reach Andros by ferry from the port of Rafina. The trip lasts about 2 hours. There are every day routes with conventional ships and high speed ferries that are more during summer period.
There is coastal connection all year round with Mykonos, Naxos, Paros and Tinos.
For more information:
PORT AUTHORITIES OF RAFINA 22940 28888
PORT AUTHORITIES OF ANDROS 22820 22250