FCDO advice including information on permitted duration of stay and visas for Sweden: www.gov.uk/foreign-travel-advice/sweden
There are various sources for information on the latest restrictions in force due to the Coronavirus pandemic:
RYA Covid-19 hub - https://www.rya.org.uk/coronavirus including advice and information for recreational boaters at https://www.rya.org.uk/coronavirus/advice-for-boaters.
Information on entry requirements can be found at https://www.gov.uk/uk-border-control?priority-taxon=774cee22-d896-44c1-a611-e3109cce8eae
Noonsite - https://www.noonsite.com/place/sweden/
Re-open EU – https://reopen.europa.eu/en
For information on the validity of RYA certificates abroad see Evidence of Competence Abroad.
There is no specific requirement for evidence of competence other than that stipulated by the vessel's flag state.
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 as it may need to be placed under customs supervision.
See holding tanks for information on the discharge of waste water.