just back from the glorious white isle and have some tips if you plan to do this - our day turned into a bit of a disaster cause we didn't know much!
If you are in San An - don't get ferry from here to Formentera!!! Costs 22 euros from front and takes 2 3/4 hrs!!! on a popeye boat - seriously, almost all passengers were sick over the side!!!
Best to get from Ibiza Town harbour - seacats run taking either 25 or 35 mins and costing 8 euros one way. So if you're in San An get the 1/2 hr bus over to Ibiza - easy peasy and only 1.40
When you are there DO NOT get on the 'Regular Bus' - it is a spanish tourist bus with 15 minute stops to take photos, personally my idea of hell!
Hope this helps!