Do I need to UKCA mark my boat if I don’t sell it?

Yes!
All the UKCA marking regulations, including RCR, require that a product be compliant when it is first put on the market or when it is first put into service in UK.  So, unless your boat is exempt for another, genuine, reason, (see more on exemptions here) it must be UKCA marked when you use it or sell it.

The only possible exception is if you build a boat for your own use. In such a case, the boat becomes exempt after it has been in service for 5 years.  See a separate note for home builds.