Megalohydrothalassophobia I believe it's called - fear of large objects in water. Or submechanophobia, the fear of submerged man-made objects.
So basically whales and submarines and the underside of boats and those really large deep tanks/diving pools. I braved google images to bring you this picture (though feel all dizzy and tense from looking at the search results!)
Thankfully these aren't things I have to deal with on a regular basis