You ha ve several choices- deep sixes or other diving devices. The problem with htese is they seriously affect the fight when you hook a fish.

You can also try poor-mans riggers. You are trolling slow enough to toss out 2-3 lb balls with rigeer release clips attached. Use 80 lb test and big rubber snubber at the boat- similar to the old fashion hand lines. Tie up a few with 60-70-80- or 90 feet of line. Other than a few weird looks and some hand over hand retrieves, this will work.