Minecarts and Primed TNT that burn cannot be extinguished by water.
What I expected to happen was:
As soon as both enter any body of water they get extinguished.
What actually happened was:
TNT still burns till it eventually explodes and Minecarts will burn underwater forever (till world reload).
Steps to Reproduce:
1. Get yourself a flame bow.
2a. Over a body of water place TNT so that when primed it would drop into the water. Shoot it with the bow, it primes but also burns.
2b. To set a Minecart on fire you have to be in Survival, shoot it and push it into the water.
3. Observe both still burn underwater (see screenshot).
Confirmed.