User blog comment:Kogath/Monster Hunter 4 Ultimate for the Nintendo 3DS/@comment-184612-20140128145845

Confirmed via Yuri from Capcom, Underwater Combat and Lagiacrus will not be returning in Monster Hunter 4 Ultimate.

http://kogath.tumblr.com/post/74833839944/confirmed-via-yuri-from-capcom-no-underwater-combat