I can relate to this. I'm not sure if it is from watching Jaws when I was five but I really hate wading where I can't see the bottom. Just as spooky are things rubbing against my legs. In San Diego I used to go to the beach at night with friends. There was nothing worse than playing in the water a wave hits you and slides sea weed along your legs. The feeling is indescribable and terrifying. Thinking about it while writing this gives me the heebie-jeebies. I become frustrated and wonder why I don't push myself past the fear. I always wonder, maybe this is a fear everyone has and if I can just push past it I will get to the fish where others were too afraid.
My last fishing trip I did just that. There was a mat of gross green underwater weeds. I guess it is called hydrilla.

When I moved through the grass I was happy that I was trying to face my fears but I couldn't wait to get to the next rock outcropping. There was one point where I was standing on a rock island surrounded by this green muck. I couldn't tell how deep it was on any side. I sort of bent down and tried to edge in with one leg. With every inch my leg went under the surface I kept hoping "please feel the bottom, please feel the bottom." Eventually my foot hit bottom and it was a lot shallower than I expected. But then came the dilemma of taking another step. I reluctantly walked through the grass and tried to not think about what was in it. I just focused on the next rocks I could climb up on. Even though I had walked through maybe 100yds of water without being able to see the bottom and what I was walking through, I was still having a hard time getting up the nerve to walk back through it. Maybe I was lucky and just stepped over a hole that I'd fall in on the way back. I psyched myself up enough to wade back through the grass and my steps would quicken the closer I got to a rock. Once I reached the edge and saw the rocks and stained dirty brown bottom of the stream I was relieved. I'm sure I'll get over my fear some day but it wasn't that day.