You could explain it by way of; they all operate in the same market so similar fixed costs, licences, insurance, maintainance etc, but it may be like the gas and electric companies, one blinks first and puts up their prices then the rest price match to them. If your competitors go up by 10% why would you only put your prices up by 5%?
I used to work the basis that a holiday on the broads or narrowboat was about 50% more than a nice static caravan in Devon or Wales so if I had some money it was a boat holiday, times not so good and we went in a caravan. Currently prices for a week on the sort of caravan we used to hire is £600 per week (in the school hoildays) so £900 for a boat would be what I'd expect to pay to hire a boat of the type we used to. I've looked at Hoseasons and like for like boat holiday prices are now double the caravan prices. So in real terms a boating holiday has gone up over the rate of a comparible, if I checked back the cost of a narrowboat holiday against 2 weeks on the Costa somewhere I suspect the difference would be greater.
The best suggestion I've seen was take a look at a share in a boat, they seem (if you have the initial investment) much the best deal.