Budget Not What is Stated
I know a few people who worked on this project and I saw a rough-cut screening in 2012. I know for a fact that the budget is nowhere near the stated $18 Million by the distributor. It's more in-line with 1/6 of that cost and when you think of the top line actors, the end result is quite phenomenal. It is also evident that the distributor didn't invest enough in the pre-release marketing effort to attract the audience it deserves on opening weekend.
Many critics like the the film, just as others do not. Budget limitations certainly constrained the story that could have been told, but the performances were all well-received and it shows in the final product. I think if people knew what the true budget was, the reviews would likely have been largely more positive. The film looks beautiful so you can see that they didn't skimp on the production values, the effects look solid so they spent what they could to make the underwater shots look real, so ultimately the film works well. And it really works well for what is essentially a taut, dramatic, period-thriller made on a slim budget.
If only the marketing was better, which could still be reversed if they were smart, Phantom could be a solid indie hit.