TnT Minecart underwater destroys blocks

While playing around with minecarts underwater I tried tnt minecarts and was suprised they destroy terain. Considering that no explosion destroyed terain underwater i find this most peculiar.

Originaly my tracks were placed on sand but I tried different materials with the same blast resistance (Ice, Gravel). It appears that the blocks the tracks are on are spared while the surounding blocks get destroyed.

Furthermore I tried it with a straight test instead of my curved track with similar results.

The only materials tried are ice, sand, gravel, dirt.