This is an excerpt from the latest ( 3 December) Q&A on the Welsh Government website. Note that the below dispensation would seem to apply to the "owner" only. Please remember that I am happy to check your boat and do any essential work for you. You have only to send me a key.
I do not live in Wales. Can I travel to Wales to secure my boat, caravan or holiday home for winter?
Under the rules in Wales, a boat, caravan or holiday home owner from elsewhere in the UK may return to collect their belongings and secure their boat, caravan or holiday home for winter, even if they live in a tier three area in England, a level three or four area in Scotland or in Northern Ireland.
If you live in one of the above areas, you should agree a time and date to visit the site. Whilst on site you should continue to observe social distancing measures. You are not permitted to stay overnight while these travel restrictions are in operation.
You will also need to check that what you are planning to do is allowed under the rules where you live, which the Welsh Government cannot advise you on.