In that scenario you can't die of dehydration but you're going to die of dehydration forcibly. So what's going to happen?

Youre going to die of dehydration, because you we're simply unaware that drinking too much flushes the sodium out of your body which is what makes you able to retain enough water to function.

Ironically people in hot environments and drinking a ton of water can end up severely dehydrated (mainly if they don't eat anything, as food has a sodium and other electrolytes).

Now if you drank mineral water (or sports drinks but they're rather sugary nowadays) or just added a tiny bit of salt to the water you drink, then it would break the prophecy.

Similarly ironic is that a lot of people who aren't used to cold environments and get lost in the woods or something usually end up suffering heat stroke, as they've only a massively thick puffy jacket and walking still generates heat, which the jacket traps and your body can't cool down and overheats. (Layers and breathing materials underneath the top layers is good, as then you can open or remove a layer as needed to regulate your body temp.)

For the sake of the topic of the thread, I'd like to know what happens if I'm told I die in 50 years from a heart attack while running a marathon, and after hearing that I jump out of a window, try to blow my brains out or shove a block of C4 up my bowels and blow myself up? I should survive, yes? And in condition to (attempt to) run a marathon?

Because if it's not locked like that and can be changed then it's more of a guess than accurate foreknowledge.