|Gark the Goblin|
Gravity damage is something the rules (even in Distant Worlds) are silent on, perhaps to account for different levels of play.
Very deep water is not only generally pitch black, posing a navigational hazard, but worse, deals water pressure damage of 1d6 points per minute for every 100 feet the character is below the surface. A successful Fortitude save (DC 15, +1 for each previous check) means the diver takes no damage in that minute. Very cold water deals 1d6 points of nonlethal damage from hypothermia per minute of exposure.Or you can use something similar to the rules for cave-ins, but harsher and more continuous.
Characters in the bury zone of a cave-in take 8d6 points of damage, or half that amount if they make a DC 15 Reflex save. They are subsequently buried. Characters in the slide zone take 3d6 points of damage, or no damage at all if they make a DC 15 Reflex save. Characters in the slide zone who fail their saves are buried.
Characters take 1d6 points of nonlethal damage per minute while buried. If such a character falls unconscious, he must make a DC 15 Constitution check each minute. If it fails, he takes 1d6 points of lethal damage each minute until freed or dead.
If going deeper into a planet's (or sun's) core, I would rule that gravity-associated bludgeoning damage scales from about 1d6 to 20d6 per round, depending on depth.
DR/bludgeoning would most likely protect against pressure-based damage. Planetary adaptation would also help to some extent.