Yachts under 24 meters normally require a European Certificate of Conformity (CE) for registration in Poland. However, if a boat is not required by law to have a CE certificate, then this requirement is waived. So we can refer to Directive 2003/44/EC of the European Parliament on the regulations of the Member States relating to recreational craft:
"The following shall be excluded from the scope of this Directive:...craft built for own use, provided that they are not subsequently placed on the Community market during a period of five years"
So, if the boat is for personal use and is not sold for 5 years since it was built, it does not need a CE certificate. It is a good idea to keep some record of when the boat was first navigable to document the start of the 5 year period off the market; A marina receipt for example. The owner will need to have some proof of ownership for Polish registration. This could be a previous local boat registration or receipts for purchased construction materials for example.
If the vessel is 7.5 meters or less, it is not required to register in Poland. But this measure is for vessels in Polish waters, there is a provision for vessels less than 7.5 meters in foreign waters to register in Poland.