When to sail between Ibiza and Formentera
The sailing season runs from late spring into early autumn. July and August need a booking. The water is the reason either way.

The months that work
The useful season for a day between Ibiza and Formentera is late spring through early autumn. The days are long, the sea is warm enough to swim, and the beaches look the way the photographs promised. Outside that window the trip is a different product, and we do not pretend otherwise.
Mornings are usually the kinder start. The day tour and the private charter are both planned as a full block of seven hours, or seven to eight on a private boat, so you are not chasing a one-hour gap in the weather.
July and August
High summer is busy on land and calmer in the planning if you book the boat before you fly. Ses Illetes will have other people on it. That is true. The difference is that you arrive from the water, swim, and leave, instead of holding a parking space.
Small-group places are capped at eight. A private boat is capped at eleven. Neither gets larger because the month is August. The rate does change. Treat the figure on the site as a starting rate and confirm it on the date you choose.
How to book the day
Pick the day tour if the plan is Espalmador and Ses Illetes from Ibiza. Pick the private boat if you want the whole deck, a Formentera departure, or Es Vedrà. Both are on the same booking page.
If the group is still deciding, book the date that is free and choose the boat that matches the headcount. The coast will still be there. The space on board will not.
Starting seasonal rates. The price is confirmed when you book.