If you run a low cost airline, then once you have paid for the purchase/lease of the aircraft, you want it to be in use every day, and to maximise the income from it. So you vary the prices according to demand: if no one is booking the price goes down until someone is prepared to pay. So in most case the flight close to full when it depart, and you have maximised the income.
As with the airline, the initial purchase cost of the boat is not directly irrelevant - it has already been incurred. From then on, it makes little economic sense for hire boats to just be sitting in the yard - at any time of year. If the hire company really wanted to maximise its income, it would be flexible on price. Setting high prices for busy periods, but in slack periods reducing prices to the level that almost every boat in its fleet is booked out at all times (allowing for maintenance). As I said above, the only critieria should be to cover the marginal costs of hire (booking cost, instruction, fuel, oil, cleaning/turn-around, depreciation etc), as after that every £ is profit. If the marginal cost is £300 for a week's hire, it is better to hire out the boat for £301 for that week than leave it in the yard earning nothing.
The downside is that the canals would be full of hire boats, every week of the year!