There are two authorities to satisfy when you travel internationally to board a cruise: one is the country of destination, and the other is the cruise line. These are two separate and distinct sets of requirements.
First, you must satisfy the requirements to enter Spain by air. IATA (International Air Transport Association) is the website that the airline employee who checks you in at the departure airport will consult to verify your documents. You can consult IATA yourself. Follow this link and click Spain on the map. https://www.iatatravelcentre.com/world.php
Sorting through Spain requirement number 1 it appears that you meet the proof of vaccination condition and need only complete and present the Spain Travel Health form: https://www.spth.gob.es/
You do not need the EU certificate, in lieu present the completed Spain Travel Health form.
Silversea requirements are here: https://www.silversea.com/lp-silversea-hrts-consumer-eu.html
"The EU requires that all embarking guests must hold proof of a valid antigen negative test result (administered by a verified third party, either a laboratory or pharmacy) performed within 24 hours of embarkation.
If you arrive for embarkation with a valid antigen negative test result (administered by a verified third party, either a laboratory or pharmacy), please make note you will enjoy expedited boarding. If you require pier-side testing by Silversea at embarkation, please note this will result in a delayed boarding process for you."
That pretty much sums it up.