if the photo is blur, and the blur is motion blur, it means that you're using too slow a shutter speed.
if the blur is all over (ie: not just moving objects like fish) then use a tripod to help
switch to manual mode and increase the shutter speed. if not enough light then the picture will be under exposed. *if mildly underexposed, you can try to fix it in photoshop*
otherwise you should be shooting with flash.
what camera are you using?
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