Dark prismarine in the new default textures has an improper outline that makes the "tiles" appear to be 3 by 3 instead of 2 by 2 (not counting the visible outline), causing weird interactions with other blocks that make dark prismarine look like it is sticking to other blocks on certain sides. This problem is also apparent when looking at prismarine stairs facing a certain direction. The programmer art texture pack does not have this problem, as shown in the screenshots.