Econo Sublaser

 This is more of a toy than the other items, but still a useful one.  The ASP Econo Sublaser is an underwater adaptation of the laser pointers used in business.  Just as with guns that use it to point at their object, the Econo Sublaser can be used for similar purposes with photography and videography.  Attach it to the camera housing (like the Econo Subcam) adjust its aim, and the red light can point to the center point of your tape.  You can also remove it and just have fun with it underwater.  It's cheap enough, though it shouldn't be thought of as a toy.  Never aim it at anyone's face.  Lasers of any type can damage eyes and can cause permanent vision loss.  Have fun, and treat it with care.  Watch where you point it.

Tools needed to build it:
adjustable wrench