Quick answer. Spain is in the EU. EU, EEA and Swiss citizens can work here with no permit. Everyone else, including UK passport holders, needs a work and residence authorisation arranged before starting.
Do skippers and crew need a work permit in Spain?
EU, EEA and Swiss citizens: no work permit. If you work or stay beyond three months you register and get an NIE (foreigner's number). Everyone else: you need a work and residence authorisation, which is employer or host sponsored, before you can start.
Does the type of yacht matter?
On a bareboat charter (up to 12 guests who charter the yacht and hire a skipper) the rules above apply. On crewed charter, private yachts and superyachts, crew are often treated as seafarers and the route can differ with the flag and where the yacht is based, so confirm for that yacht. Working ashore follows ordinary work-permit rules.
Official source
Spanish Ministry of Inclusion, Social Security and Migration (immigration) and the Ministry of Foreign Affairs (visas). Confirm the current rules for your nationality before you travel.
What we ask of you
Keep your nationality correct in your profile; it tells us your EU status.
If you hold a work permit for Spain, tick the work-permit box.
Tell us early if your right to work is not sorted, so we can help before your dates.
Important
Immigration rules change and vary by nationality. This is general orientation, not legal advice, and Quarterdeck is not an immigration adviser. Confirm your own position with the official source above before you travel. We will not place you in Spain if you do not hold the right to work. See also: Visas & work permits, overview.