I have a visual impairment how do I know where my blade is in relation to the water?

There are many exercises that coaches use with crews that are relevant to individuals with visual impairments - simple placement drills and 'slapping' the water before the catch will all help you (whether you have full or limited sight) become more aware of where your blade is in relation to the water at the catch. Additional aids would be to have straws in the boat at the catch indicating where you need to raise your hands to - providing a physical stimulus as well as kinesthetic feel.