The Ocean Expansion Pack allows you to inject one of two cobs which add new rooms and objects to the dark blue ocean. There are two parts to this cob: The first allows you to inject water-filled rooms for your aquatic norns (ie. MerNorns), and the second allows you to inject air-filled rooms that any norn can use. Besides this there are no real differences between the two cobs.
Using the Ocean Expansion Pack in the same world with Spriglysium Nebula is not recommended.