FCDO advice including information on permitted duration of stay and visas for Sweden: www.gov.uk/foreign-travel-advice/sweden
For information on the validity of RYA certificates abroad see Evidence of Competence Abroad.
A valid certificates is required for operating a personal watercraft (PWC) in Sweden. Foreign citizens operating a PWC must be over the age of 15, must carry international ID or their passport and have a certificate valid for PWC issued by their national authority.
EU member state (part of the customs territory of the EU (which includes territorial waters)). Further information is provided on the Entry & Exit Formalities page under Customs - EU Member States.
Part of the Schengen area.
As part of the customs territory of the EU (which includes territorial waters) and the Schengen area customs and immigration checks may not always be required on arrival and departure.
Non-nordic citizens arriving in Sweden other than from a Nordic country must notify a passport control centre.
If your boat is in Sweden under Temporary Admission and you need to leave Sweden but the boat is remaining there, you should notify customs.
See https://www.tullverket.se/ for further information.
See holding tanks for information on the discharge of waste water.