If you have permission before hand I would use the flash.
Most athletes should be use to flashes from the various athletic events with parents and relatives present. Or if they move on up the press will be there to do it.
Try too use a bounce on the flash that should minimize any harsh lighting.
Our boys played football, basketball, baseball and etc. And I was always there using the flash along with many others. If you get permission check with each diver and see how he or she feels about it and if there is any critical point in their dives they would prefer you not use the flash. Try to time it so their eyes are not looking towards you?
The diving board is always the same distance from you ( as long as you don't move ) so I would just try to use manual focus, the subject are also most often the similuar size so your not zooming in out every time, you might want to try to use 50mm 1.4 / 1.8 , to work in these poorly lit situation.
With this particular shot I think you have did a great job, I like it :)
I can tell you from experience from the athlete's perspective that flash is not that distracting! I'm a competitive swimmer, and have had many a flash go while diving, or swimming, and I pay little attention to it! Anytime there have been photographers at the pool, professional ones, they've used some major flash with no problems! The only rules are that you can use it during the start, as a flash is used for the timers, other then that it's free reign!
The diving board is always the same distance from you ( as long as you don't move ) so I would just try to use manual focus, the subject are also most often the similuar size so your not zooming in out every time, you might want to try to use 50mm 1.4 / 1.8 , to work in these poorly lit situation.
ooopss, forgot to see what type of camere you ware using :)
It would of course only apply to DSLR
