At the very least there is a reasonable expectation of professional results, even if those results end up being nothing. For any amount of money, certainly for just south of $1M, you don't expect for full equipment failure without reasonable backup equipment nor do you expect that the coordinates of the search are any other than correct. For a total system failure there is no excuse. For having submitted the sonar results as a single piece - thus providing incorrect data - and not correcting their results to include the dead spots of the sonar data again shows incompetence or lack of understanding of their job.
I would be surprised if you got a 100% refund but based on the data gathered and the performance of the company I would expect a significant portion of it.