Every minute that a character is out of his/her depth in water, he/she must make a dice roll using his/her CO + Swimming (including Swimming strength/Staying afloat specialisms) or he/she will sink below the surface.
The difficulty value depends on the conditions as follows:
| Conditions | Typical difficulty |
| Placid, calm conditions (e.g. lake, river) | 0-2 |
| Slight waves (e.g. calm sea) | 3-6 |
| Significant waves (e.g. rough sea) | 7-10 |
| Stormy conditions | 11 or more |
There are modifiers to the basic value for staying afloat, depending on the way in which the character is swimming:
| Swimming style | Modifier |
| Treading water | +4 |
| Crawl | 0 |
| Breast stroke | +2 |
| Back stroke | +1 |
| Butterfly/Sprint crawl | -2 |
For example, Roger (an expert swimmer with a Swimming skill score of 5, 3 points of specialism in Swimming strength, 2 points of specialism in Staying afloat, and a CO of +1/2) is swimming unencumbered (+2 to dice rolls using CO) using the crawl. His basic value is 5+ 1 1/2+ 2+ 1/2+ 2 +0 = 11, which gives him a certain dice roll result of 11, meaning he can automatically stay afloat in even a rough sea. When lifesaving Charles (see above), he is instead swimming backstroke and is heavily encumbered (- 5 1/2 to dice rolls using CO), changing his basic value to 5+ 1 1/2 + 2+ 1/2- 5 1/2 +1 = 4 1/2, which gives a certain dice roll result of 4. Under normal conditions, Roger can automatically keep himself and Charles afloat, but if the conditions deteriorate, Roger may have to switch to treading water to keep them afloat or start taking risks on his dice rolls.
Holding on to a float (e.g. piece of wood, air-filled bladder, etc.) adds between +2 and +4 modifier to the dice roll for staying afloat, at the referee's discretion.
If a character fails the staying afloat dice roll, he/she will sink below the surface. He/she will sink at a rate of one metre every 5 seconds. Every 5 seconds, he/she can attempt to stop sinking and rise to the surface. Each 5 seconds that he/she is trying to rise to the surface, he/she will rise by one metre rather than sinking by one metre if he/she succeeds in a dice roll using CO + Swimming (including Swimming strength/Staying afloat specialisms) at a difficulty of 6.
Normally, characters who have sunk below the surface will tread water to get back to the surface (and so gain the +4 modifier to their basic value for this). Characters who have sunk below the surface often jettison equipment to increase their effective CO as well.
When the character regains the surface, he/she can start swimming normally again (though of course, he/she has to keep making the staying afloat dice rolls every minute to stay this way). A character who runs out of breath while below the surface will take damage from drowning (see Holding breath).