I had a nose bleed once. The diagnosis was obvious: "Nose banged on a rock".
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Thanks for all the responses. The bloody nose had a little bit of a flow to it, so he might have had sinus because the weather just changed in my neck of the woods. How do you go about taking care of that under water?
I had a bad nose bleed once (not underwater). A nurse told me to pinch my nose for 3 minutes (it was just pouring out). Where you pinch is important. You want to pinch your nose at the highest point that still closes it off. Don't just pinch the bottom. Pinch as much as you can. How long you have to pinch depends on how much it is flowing and how well you clot.
I'd also call the dive at that point. There is a LOT of blood flow in the face. You can quickly bleed out enough to pass out or at least get lightheaded and dizzy. Mind you, if it started bleeding as bad as the one nose bleed I had, it would fill your mask in a few minutes.
Bloody noses are quite common on the first few dives that a new diver makes. I have seen as many as a third of the students on their first open water dives come up with some blood in their masks. It becomes less and less common with additional dives. It almost never happens after ten or twelve dives. This is pure supposition on my part, but I think that the weak capillaries break when put under strain on the first couple of dives and then scar over and do not bother you again.