It is not necessary to read the "Dirk Pitt" novels in order. They can be read in or out of chronological order, but it is not essential in order to follow the character through his adventures.
As of 2015, there are 23 "Dirk Pitt" novels. However, to read them in chronological order, it is recommended to begin with "Pacific Vortex," which was first published in 1983. Downloadable PDF files are available at various places online as well as sites that list the novels in chronological reading order. It should be noted that the publication order does not follow chronological order.