String once bubbled...
One thing to mention when on about the knots are if there is any sort of current the line wont be vertical so the knots wont be at the depth you expected.
Well...yes and no. If you are using an upline tied, for example, to a wreck, then the bag at the other end of the line will drift down current and you will need a lot of lift to keep the line relatively vertical or else you will need to pay out more line for a more angled ascent. Which also reaches a point of diminishing returns as the longer line imparts more drag and pulls the bag farther under.
But with regard to knots we are talking about knotting the end of the line on your reel 10 or 20 ft from the end. (you can't put a knot 20 ft under the surface on an upline for obvious reasons. ) And with a reel you will be free floating under the bag and there will be very little deviation from the vertical, so the knot will be very close to where it is supposed to be.