This is covered by the Falling rules on, appropriately enough, Falling into Water:
Falls into water are handled somewhat differently. If the water is at least 10 feet deep, the first 20 feet of falling do no damage. The next 20 feet do nonlethal damage (1d3 per 10-foot increment). Beyond that, falling damage is lethal damage (1d6 per additional 10-foot increment).
Characters who deliberately dive into water take no damage on a successful DC 15 Swim check or DC 15 Acrobatics check, so long as the water is at least 10 feet deep for every 30 feet fallen. The DC of the check, however, increases by 5 for every 50 feet of the dive.
In the three scenarios—boldly assuming the ocean in question is at least 10 ft. deep—, the following could happen:
- An 80 ft. fall deals 2d3 points of nonlethal damage and 4d6 points of damage. That is, no damage for the first 20 ft., 2d3 points of nonlethal damage for the next 20 ft., and 1d6 points of damage for each of the remaining 10 ft.
- An 800 or 8,000 ft. fall deals 2d3 points of nonlethal damage and 20d6 points of damage. That is, no damage for the first 20 ft., 2d3 points of nonlethal damage for the next 20 ft., and 1d6 points of damage for each of the remaining 10 ft. up to a maximum of 20d6.
While it's possible the creatures could dive into the water 80 ft., 800 ft., or 8,000 ft. below, succeeding on the Acrobatics or Swim skill check so as to avoid being dealt damage is increasingly challenging:
- The 80-ft. dive requires water that's at least 20 ft. deep and mandates a skill check (DC 20).
- The 800-ft. dive requires water that's at least 260 ft. deep and mandates a probably impossible skill check (DC 95).
- The 8,000-ft. dive requires water that's at least 2,660 ft. deep and mandates what will probably end up being the campaign's most impossible skill check (DC 815).
In other words, good luck if your PC's making the 800-ft. or 8,000-ft. dive.