Whipper Snapper,
All of the replies here listing schools are right on the mark. As archaeology is a sub-discipline of anthropology, I would suggest that you start boning up on general anthropology. I recommend reading "Anthropology" 9th or 10th editions, Haviland, as a primer to Anthropology and all of its sub-disciplines. I am 45 years old and have been interested in becoming an "Archy" in my old age but don't have the time to go to college and I can't find online courses in Anthropology or Archaeology (I live in Egypt). I would also suggest that you take a course designed to train non-Archy's to assist trained Underwater Archy's in conducting surveys and excavation of underwater cultural heritage. Learning a second, or even a third language can be extremely useful when conducting archival research. Language to learn depends on which era, period, and/or geographical location that you decide to specialize in (Ancient Greek for the Mycenean and Hellenistic periods, Ancient Egyptian for Egyptology, Dutch for the VOC ships, Spanish for South and Central America,for example). A couple of Geology courses, Chemistry, Biology, and Statistical Analysis. Since I'm not going to college at this time, I read all of the books that I can get my hands on based on the required reading lists from various colleges. I have Archy friends here in Egypt as well, that tell me what I need, or don't need. I'm also am sure that there are others here that are more qualified than I to give you advice here. In the end, just by reading and learning about Anthropology and Archaeology on my own, it has given me a greater appreciation of what I'm looking at and finding when I drop in on any one of the myriad of wrecks and historical sites that we have here on the North Coast of Egypt (Pharonic to Post-modern sites). Reading books by George Bass of the INA, Ballard (of Titanic fame), Mel Fisher, and others like them, are also of great help in understanding the processes involved in the research of, survey and excavation of, and conservation and preservation of underwater cultural heritage. So, if you do decide to become and Underwater Archy, I wish you all the best in your endeavors.
Mike