Complex sentence: Although the base was moved to an asteroid, early plans for "Arrival" involved it set on an ocean planet--the base would be underwater and Shepard would reach the main level by submarine.
Split into simpler sentences: Although the base where most of the pack's events take place is located on an asteroid, early plans suggested that it would be set on an ocean planet. The base would be underwater and Shepard would reach the main level by submarine.