There's a 20% discount, confirmed by this page (not available in Spanish, it seems). However, they don't say how to book with this discount.

So please ask the ferry company how to book this. At the bottom of that page, there's an option to fill in a contact form or to contact via Whatsapp.

Remember that you don’t need to activate a travel day for a discount.

Flying will be cheaper-if booked some time advance. Sadly its that RYANair, also check Vueling-so do read all conditions etc. ESpana is much notable for incomplete, outdated or other false info, also for their trains

For the boat: it usually means you have to go yourself to the office of THAT ferry=Balearia and buy the ticket by showing pass-there is usually no need to pre-reserve monthes advance. That is only for people in cars. On a small island that should not be overly hard to do. Plus that you also have to get there -you will not go swimming, I suppose?