I would strongly suggest you get your ICC as not all Spanish organisations recognise RYA qualifications.
If you spend some time sailing around Spain you'll notice that rules/regulations are not always applied consistently and the outcome often depends on the person/organisation you are dealing with.
It's not just the company you are chartering the
boat from but also the Guardia Civil (Spanish Police - both on land and at sea) that you may have to interact with at some point in your holiday.
For additional context, the RYA is a British organisation and some Spanish authorities will take the opportunity to remind you that Great Britain is no longer part of the EU - especially if you are sailing near
Gibraltar which the Spanish think should be theirs!
I have experienced this firsthand. I have my
RYA Yachtmaster Offshore (commercially endorsed) yet am required to carry my ICC when sailing in Spanish waters (you only need RYA Day
Skipper to get an ICC).
I'm not sure of the process to get an ICC for ASA certifications but suspect it's as simple as for RYA qualifications where it can all be done online.