You are not able to pack dirt that is submerged in water, however it appears that packing dirt with one edge submerged is easier on South-East facing tiles, While NW facing tiles need to be completely raised from water to be packed.
With Body strength higher than 21, you are able to pave a tile, then dig it into water to completely submerge it. This technique appears to be commonly used for Ship docks/Harbours to ease the exiting of the vessel onto dry land.
[[Mycelium]], [[moss]] and [[enchanted grass]] tiles can be changed to packed dirt with a shovel.