I don't know if anyone knows of a fairly renowned herpetologist named 9 finger Mike who lost a finger to a stilleto bite - and is working on a show similar to River Monster's in which he will be giving expert advice to whatever TV face friendly jerk off they have wrangling the animals - but straight from his mouth were the words that fly river turtles absolutely and 100% need UVB.
He recommends UVB 10.0 bulbs directly over open waters, or to get an aquarium specially made with the intentions of a turtle that has one half screen, though screen will also filter out UVB.
Fly river turtles don't need as much UVB for shell production and calcium transfusion as other turtles that bask, but they still need it - and will swim closer to the surface of the water to get it.
This is from the mouth of a pHD who has dedicated 30 years of his life to reptiles, not from me - he comes into our shop regularly to talk with us because the studio he works in is right down the street.
Without it they will not die, but will develop a soft shell condition due to calcium deficiency that leads to an easier invasion of bacterial and fungal infections.
So while it's not critical for their survival, it is an important part of their care - if you want your animal to have the best care you can provide, which as a hobbyist is your obligation.
2 hours in natural sunlight is equal to 8 hours of artificial flourescent UVB as well - they also make drops that eliminate the need for UVB, but the directions are "per inch of animal" which is completely inaccurate, as a result you run the risk of crystalizing your animals organs.
If you see fit you can visit a zoo and get exact gram measurement calculations for giving your turtle artificial calcium infused with certain catalysts to simulate what UVB would let them do naturally - but it seems like a lot of extra work, when you just need to replace a bulb every 6 months.
Just my opinion.
EDIT:
Also - don't put basking/UV bulbs that are mogul socket screw ins over a tank, that's a recipe for disaster. A splash of water could make the bulb explode - and most contain mercury which would likely kill or at the very least shorten the life span of your animal if it falls into the water.
Get the T8 or T12 (depending on length of bulb - T8's are 12" - 26", T12's are 36" and above.) fluorescent tubes in 10.0 spectrum for maximum penetration, and a regular strip light aquarium light.