Is this normal? What kind of phobia is it?

This may be a little strange, but I have a huge fear of submarines and deep water. I love to swim and I love going into shallow water, but as soon as I go in deeper I get very afraid. not that i'm going to drown, but because I don't know what's underwater. Also, when ever I hear the word 'submarine' or 'topedo' I get a weird feeling in my tummy, and I feel so scared when I see a photos of them or having anyone talk about them. I hate this feeling. I really want ot get over my fears. How can I? Also, what is this called?