Swimming

All characters are either untrained swimmers or proficient swimmers.
When the DM determines the swimming ability of characters, the decision should be based on his campaign. If the campaign is centered around a large body of water, or if a character grew up near the sea, chances are good that the character knows how to swim. However, being a sailor does not guarantee that a character can swim. Many a medieval mariner or black-hearted pirate never learned how to swim and developed a morbid fear of the water! This is one of the things that made "walking the plank" such a fearful punishment. Furthermore, some character races are normally suspicious of water and swimming. While these may vary from campaign to campaign, dwarves and halflings often don't know how to swim.
Untrained swimmers are a fairly hapless lot. When they are unencumbered, they can manage a rough dog-paddle in relatively calm waters. If the waters are rough, the current strong, or the depth excessive (at sea or far out on a lake), untrained swimmers may panic and sink. If weighed down with enough gear to reduce their movement rate, they sink like stones, unable to keep their heads above water. In no way do they make any noticeable progress (unless, of course, the object is to sink beneath the surface).
Proficient swimmers are able to swim, dive, and surface with varying degrees of success. All proficient characters are able to swim 1/2 their current land movement rate times 10 in yards, provided they are not wearing metal armor. A character with a movement rate of 12 could swim 60 yards (180 feet) in a round. Characters whose movement rates have been reduced to 1/3 or less of normal (due to gear) or who are wearing metal armor cannot swim — the weight of the gear pulls the character under. They can still walk on the bottom, however, at their current movement rate.
Proficient swimmers can double their swimming speed, if a successful Strength check is rolled (vs. 1/2 the character's normal Strength score). For a character with a movement rate of 12, a successful check means he can swim 120 yards in one round, an Olympic-class performance.
Like running, swimming is not something that a character can do indefinitely. There are several different speeds a character can choose to swim at, thus moving in either short sprints or a slower, but longer-lasting, pace.
• If swimming at 1/2 normal speed or treading water, the character can maintain this for a number of hours equal to his Constitution score (although he will have to abandon most of his gear). After a character swims for a number of hours equal to his Constitution, a Constitution check must be made for each additional hour. For each extra hour of swimming, 1 Constitution point is temporarily lost.
Each hour spent swimming causes a cumulative penalty of -1 to all attack rolls. All this assumes calm water. If the seas are choppy, a Constitution check should be made every hour spent swimming, regardless of the character's Constitution. Rough seas can require more frequent checks; heavy seas or storms may require a check every round. The DM may decide that adverse conditions cause a character's Constitution score to drop more rapidly than 1 point per hour.
If a swimming character fails a Constitution check, he must tread water for 1/2 hour before he can continue swimming (this counts as time spent swimming, for purposes of Constitution point loss). A character drowns if his Constitution score drops to 0.
As an example, a freak wave sweeps Fiera (an elf) overboard during the night. Fortunately she can swim and knows that land is nearby. Bravely, she sets out through calm water. Her Constitution score is 16. After 14 hours of steady swimming, she makes out an island on the horizon. Two hours later she is closer, but still has some way to go. During the next hour (her 17th in the water), her Constitution drops to 15 (her attack penalty is -17!) and she must make a Constitution check. A 12 is rolled — she passes. In the last hour, the 18th, the seas become rough. Her Constitution is now 13 (the DM ruled that the heavy seas made her lose 2 points of Constitution this hour), and the DM decides she must pass an extra Constitution check to reach shore. She rolls a 5 and flops onto shore, exhausted.
• Characters can also swim long distances at a faster pace, although at increasing risk. Swimming at the character's normal movement rate (instead of the usual swimming speed of 1/2 the normal movement rate) requires a Constitution check every hour, reduces Strength and Constitution by 1 point every hour, and results in a -2 cumulative attack penalty for each hour of swimming. Characters can swim at twice this speed (quadruple normal swimming speed), but they must roll a check every turn and suffer the above penalties for every turn spent swimming. When an ability score reaches 0, the character sinks and drowns.
• Upon reaching shore, characters can recover lost ability score points and negate attack penalties by resting. Each day of rest recovers 1d6 ability points (if both Strength and Constitution points were lost, roll 1d3 for each ability to determine points recovered) and removes 2d6 points of attack penalties. Rest assumes adequate food and water. Characters need not be fully rested before undertaking any activity, although the adjusted ability scores are treated as the character's current scores until the character has rested enough to fully recover from the swim.
To continue the earlier example with Fiera, after a bad last hour in the water, she reaches shore. Her Constitution is 13 and she has a -18 penalty to her attack roll. Exhausted, she finds some ripe fruit and collapses in the shade of a palm tree. All the next day she rests. At the end of the day she rolls a 4 on 1d6 and regains 4 points of Constitution, restoring it to normal. An 8 is rolled to reduce her attack penalty, so the next day she suffers only a -10 penalty to her attack roll. The next day of rest lowers this by 6 to -4 and the third day erases it completely, so in three days she has fully recovered from her 18-hour ordeal in the water.

Holding Your Breath

Under normal circumstances (with a good gulp of air and not performing strenuous feats), a character can hold his breath up to 1/3 his Constitution score in rounds (rounded up). If the character is exerting himself, this time is halved (again rounded up). Characters reduced to or less of their normal movement because of encumbrance are always considered to be exerting themselves. If unable to get a good gulp of air, these times are reduced by 1/2. All characters are able to hold their breath for one round, regardless of circumstances.
While attempting to hold his breath beyond this time, the character must roll a Constitution check each round. The first check has no modifiers, but each subsequent check suffers a -2 cumulative penalty. Once a check is failed, the character must breathe (if he cannot reach the surface, he drowns).
Diving: All characters can dive to a depth of 20 feet in a single round. For each encumbrance category above unencumbered (or for each point of movement below the character's normal rate), two feet is added to this depth (the additional weight helps pull the character down). A short run or a few feet of height adds 10 feet of depth to the first round of a dive. For every 10 feet of height above the water, an additional five feet of depth is added, up to a maximum addition of 20 feet. Thus, with a run and from a height of 40 feet or more, an unencumbered man can dive 50 feet in a single round.
Surfacing: A character can normally rise at the rate of 20 feet per round. This rate is reduced by two feet for every encumbrance category above unencumbered or for every point of current movement below the character's normal rate Note that heavily loaded characters (those who have lost 10 or more points off their normal movement rate because of their current encumbrance) cannot even swim to the surface. Those simply floating to the surface (unconscious characters, for example) rise at a rate that is five feet per round slower than someone similarly encumbered who is actively swimming up to the surface.
It is quite possible for a moderately weighed-down character to sink if he makes no effort to stay on the surface.