To add to Paul's good information, the ferry that connects Santorini with Naxos operates daily. Tinos is not on the same ferry route that connects Santorini with Ios, Naxos, Paros and Athens/Piraeus. Neither is Mykonos, which is on a different highspeed ferry route that includes Crete, Santorini, Ios, Paros and Mykonos. Tinos IS on the ferry route that includes Mykonos, Andros and Rafina on the mainland (the alternative to Piraeus).
Once the schedules are published by the various ferry companies you will be able to see them all here:
http://www.openseas.gr/OPENSEAS/index_en.vm
To get an idea of what to expect next summer you can also use the Openseas website's calendar function to backtrack to last summer to see what was available then. Use "Piraeus" for Athens.
Schedules for May-June should be out by mid-April. Some already are. For the Piraeus>Santorini schedule go to
http://www.bluestarferries.com
Click on 'Domestic Lines' then select 'Cyclades' and 'Itineraries'
Here is a link to the schedule for the highspeed ferry that connects Santorini with Mykonos. Take note of the no-sail Wednesdays:
http://www.ferries.gr/hfd