Our experience is that hire companies will do their best to keep the customer satisfied.
We had booked a boat with a company we had been to before, we like their boats and the routes, we knew that this boat was still being fitted when we booked. A few days before our holiday began, they emailed us to say that the boat would not be ready for us, but they offered another, bigger boat at no extra cost. When we arrived, that boat was not ready as well, they had decided to put in a new kitchen, but that took longer than planned. They offered us many apologies and made sure that the boat was ready early in the evening. As a gesture of goodwill the company paid for the groceries we had ordered through them and they even threw in a couple of bottles of wine. Early the next morning we set off. We returned to that company three or four times after this.
In the year of the floods (2007?) we booked a boat on the L&L. Before we came over (P&O Ferry to Hull, an expensive crossing), we kept in touch with the company, if there was no chance of moving a boat due to the floods, we were not going to come to the UK, but they said to come over and see how things were at that moment. The boat we had booked was stuck in Stanley Ferry, we were given two options: go to the boat and see what happens, but the chances were that we would not be able to move during the next two weeks, we said no to that. The other option was that they would take us in a mini-van to a place that was over an hour's drive away, where another boat of that company was available. We gladly accepted that offer and had a good (although wet) holiday.
Ok, sometimes you end up on another boat than the one you booked, but that is not the end of the world and in both cases we could see the reasons why it happened and the way the hire companies reacted was very pleasant and professional, both wanted us to have a good holiday and did everything they could to make that so.