A sudden mud storm
While mud isn’t particularly a cause for concern, a mud storm can be. Imagine diving peacefully underwater when mud suddenly flies from the ocean’s floor and covers you in a dark, vision-reducing cloud. That’s exactly what happened during one of the OP’s diving trips.
The most extreme cause for mud storms is hurricanes, which pull up mud and everything else at the bottom of the ocean. That wasn’t the cause in this case, but mud storms that occur from out of the blue water are definitely not something you’d want to find yourself in. Thankfully, the worst the mud storm did was cloud the diver’s vision.