In the case of a new boat the broker could be but is not necessarily the distributor , possibly the broker could be the importer
The new boat may be ''placed on the market'' before the boat has been constructed but RCR requirements must in place before the boat is placed on the market.
Then the boat is built and delivered (lets say this is in the UK) and sold and must include the CE plate and relevant documents.
RCR requirements are then 100% satisfied.
The boat may be sold as used any number of times while remaining in the UK with no activity with respect to RCR.
If the boat is not exported or significantly altered there is no further RCR activity indefinitely.
I dare say the original RCD/RCR documents would help in a PCA situation for export. But in practice he costs are prohibitive because there are no grandfather rights at the present time. So a used boat would have to comply with emission standards for example . So export or import of a used boat is effectively prohibited by cost.
Alan, you said yourself that you did not need the RCD evidence when you imported your yacht (presumably from the EU prior to Brexit) and it came under government scrutiny for VAT.
The term 'placing on the market' refers to new goods
https://www.gov.uk/guidance/placing-manufactured-goods-on-the-market-in-great-britain#:~:text=A product is placed on the market when an offer,physical transfer of the product.