Is working in a diving center frequented by foreigners considered somewhat unlawful (haram)?

The default in dealing with disbelievers is permissibility. However, if working at the center aids in something forbidden by Sharia, such as the mixing of men and women or the uncovering of 'awrah (private parts), then the work becomes forbidden, and it is not permissible to remain in it. This is due to the assistance it provides in committing forbidden acts and exposing oneself to temptation. One should seek another job that is safer for one's religious commitment.