Because I intended moving to Spain, as a resident, I had been in touch with Spanish Legal Authorities to find out the situation with bringing my boat into the country for UK citizens.
If you are just sailing into Spain and intend being in Spanish Waters
for less than 183 days = 6 months in any year:
then really you have no problems.
You must carry on board your registration certificate (SSR or 'Part 1' for UK boats) your passports, a copy of your insurance documentation and a VHF operators licence. These rules apply to all EU countries for a EU registered boat with EU owner/skipper.
No foreign country can dictate what qualifications or safety equipment a yacht or skipper in their waters must conform to except it should conform to the rules of the country in which the boat is registered. (The UK does not require any qualification to 'drive' a boat so you do not need to carry one.) You may choose to, because not all Spanish Customs/Guardia Civil know that is the UK law! To have a ICC or Yachtmaster is useful. Authorities from any EU country can require 'proof' that VAT has been paid on the vessel.
If you have property in Spain and a boat in a local port but are personally in Spain for less than 183 days - 6 months of the year, then you also do not have a problem. It is your 'non residence', not the fact the boat is in Spanish waters that counts. It is sensible to have the boat 'precintado' or sealed by Spanish Customs when you are 'away' in order to prove the less than 183 days of residence/use of the boat. (The onus of 'proof' is on you to prove you were not in the country - it is not on the Spanish Authorities!)
If you intend sailing into Spain and spending more than 183 days = 6 months in one year - even if you leave and re-enter during any 12 month period, then different rules/laws apply:
If you are land based or living in your boat in Spain, for more than 180 days in any 12 month period, then the Spanish Authorities will consider you to be resident in Spain. (You will remain domicile in your country of origin (e.g. UK) but for Spanish legal and tax purposes you are considered to be a resident of Spain.
(The same rule applies to France, Netherlands, Greece, Portugal and other EU countries)
If you own a property in Spain and spend more than 183 days = 6 months in Spain in any 12 month period, then the Spanish authorities consider you to be resident in Spain
THIS IS HOW THE SPANISH LAW EXPECTS RESIDENTS TO BEHAVE WITH THEIR BOATS:-
You must pay 'Matriculation' tax of 12% of the value of the boat.
However it is possible to have this Matriculation' tax of 12% waived if you apply for Spanish registration as follows
1.The registration must be applied within the first 30 days from becoming resident in Spain.
2. The boat should have paid the standard taxes in the EU country of origin.
3. The boat should have been owned by you for at least 12 months before of getting the Spanish tax residency.
The ownership who obtain this tax benefit cannot sell the boat before six months.
If it is not possible to prove that VAT has already been paid then you would need to pay VAT. (You should be able to prove that it has been paid fairly easily from the boats paperwork)
You will be liable for the annual Tarifa G-5 tax which is effectively a tax on having a boat and varies with the power of the engine and the size of the boat. Bit like the old 'light dues'. - the same tax exists in France and several other EU countries. It is payable locally. The 'average' cost for a 12 metre sail boat is probably around 600€ annually. This tax appears to be applied in Valencia but not in other parts of Spain. It is sometimes charged by the marina and included in the berthing fees.
The law in Spain changed in January 2011 and it is no longer required that EU citizens resident in Spain re-flag their boats, so once you have applied for the exemption of the matriculation tax, as in many other EU countries, you will be required to pay the same boat tax as Spanish Citizens. You will be allowed to equip and sail your boat according to the laws of your country of origin.
For UK citizens there are no legal requirements on boat equipment or certificates to allow you to drive your boat, so the same applies in Spain.