Apparently liquids can't replace floor blocks if it would place them next to a block of the same liquid, thanks to isBorder(), but I think this should be allowed.

Presently, a 1.0 (100%) chance to replace underground prismarine floors with water results in a checkerboard of water and prismarine (50% water).

842b63a didn't help as far as I can see. In a new world with r1.6-177, prismarine cave floors are still checkerboarded at best.

Vazkii agrees, so i'll go do this