Over the years, I have successfully fished as deep as 150 feet with soft plastics. Getting down to where the fish are is a matter of weight and overall shape, especially when you're trying to reach a deep reef or wreck. If there is current or the boat is drifting with the wind, it takes more weight to get down quickly. The most effective designs for this are streamlined: Usually the choice for deep work is an arrow-shaped jig-type head with a slender cylindrical body up to 14 inches long, such as the RonZ 4x series. Some baits have a twisted tail to provide swimming action, which often gets more strikes, especially in colder water.
The basic technique is simple. Drop the lure over the side or cast it a short distance, and let it sink with the bail open or in free-spool. As soon as it reaches the desired depth, immediately start retrieving it with lots of rod-tip action, shifting from short, quick upward sweeps to longer, somewhat slower lifts. Vary the retrieve rate until you find the right combination. If you are not getting as deep as you need as fast as you feel you should, increase the weight of the head.
On a calm day with no current, you might get to over 100 feet with a 2-ounce lead head. Other times you may need 3 ounces or more. For maximum weight, add egg sinkers to the leader above the jig head.
I have caught dorado and tuna on the surface by always having a second rod rigged with a slow-sinking softy suitable for quick casting. And very often, just drifting and slowly jigging a soft plastic with little or no weight behind the boat is exactly what's needed for fish cruising or feeding at mid-depth levels, especially when you're chumming or chunking.
Keep in mind that offshore fish range from big to extremely big and are therefore strong, so use heavy-wire hooks like those offered by Shankas and Hogy.