User blog comment:Evans0305/BioShock 2 Concepts: Aqua Splicer/@comment-72.20.94.124-20110409181732/@comment-1330314-20110409225042

I think they actually did want to include underwater combat at some point, but rejected it because of all the problems you mentioned. Also, they wanted to create "100% atmospheric" areas, where you could just explore your surroundings without being distracted by combat.