It due to the fact that people are staying home and looking for a way to spend their leisure time, instead of going abroad. Too many people and not enough boats for the demand is giving greedy brokers and owners license to inflate prices by around 20%.
The same is for the caravan and camper van market. Staycation properties are the same. All driven by greed and the ready money market we have at present. A work colleague of my wife had a cottage booked in north Devon for august, they booked it in December 2029, in March this year their booking was cancelled, apparently due to Covid (the old new excuse for everything). They paid £1200 for 10 days. Money was refunded, but a month later the same place was being advertised for x3 the amount for the same dates!
pure greed.