For open water otters use a bucket set. It basically mimics a bucket set used for raccoon with a 220 conibear guarding the end of a square bucket. Take a softball sized rock, wrap it in aluminum foil and place it in the bottom of the bucket. Set the bucket UPRIGHT in about waist deep water. The otters swimming the pond almost always investigate the shiny rock and swim down into the bucket.

I have caught dozens of otters this way and there is no worry of accidental catches. You also can wire a fish to the bottom of the bucket. The white bucket stands out in the pond setting and otters cant resist investigating. You can youtube bucket set trapping if you have any questions about how to do it.