You can create a category, but think through the purpose of the category, and also who you might want to collaborate with. I'd want some feedback from the Military and War Project before creating any military related category.
Overall, though, a category is a convenience that groups profiles for a useful purpose. First of all, I don't think there is a genealogical purpose in grouping divers together -- if you discover two people are both divers, it doesn't make it more likely that they are related to each other (but maybe DNA will tell us a particular gene makes a person want to be a diver!) But there is a human interest factor in grouping divers together.
Now, do you want to limit the category? The person you're profiling probably already has a category placing him or her in the US Navy. Do you want to group only US Navy divers, or all military divers? Would your human interest in grouping divers together be helped or hindered if the grouping that your category makes included some British or Australian -- or even Russian military divers?
And as a veteran I do express my own inclinations there -- I suspect that there are commonalities in military divers that are not shared with civilian divers. But I'm not a diver so I could be quite wrong in that.
These are the kinds of things one thinks about in categorization!
Categorization Project Coordinator