We used to do this topside and underwater for recording cracks found using magnetic particle inspection; from the sound of it you are doing much the same thing but using a fluorescent dye penetrant to reveal the crack, however teh process should much the same.
The set-up we used was a 35 mm camera (this was some 30 years ago when we worked on this) with 35 mm or 50 mm lens and diopter - you could use extension tubes, bellows or a dedicated macro lens, just ensure that the set up gives you a resonably sized image. the lens was set at infinity and the camera was fitted with a stand-off frame, basically two prods which ensured that the camera was the correct distance from the object so no need to struggle focussing in poor light. Illumination was from a regular strobe fitted with a filter that blocks visible light but allows UV to pass (possibly a R72 but I can't recall for certain and i don't have the technical article we produced to hand).
I don't see any reason why this should not work with a digital camera since it is essentially recording visible light give off when the dye is illuminated with UV. If you don't want to go to the bother of using a strobe then a jig of some sort to mount the camera so that the subject pieces can just be placed under it and use a continuous UV source. The main advantage of using a strobe was that the equipment could be easily set up topside and then passed to a diver who just had to position it and press the trigger.
We found that we did not need to use very high ISO film, I think most of the work was done with 200 or 400 ISO.
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