Swimming in Rulings

The rules for swimming and drowning in B/X are fairly nebulous, reducing movement rate to half and leaving the probability of sinking and drowning to the Referee - citing only the example that a character overburdened by treasure and wearing heavy armor "might have a 99% chance of drowning" - where the same character, wearing leather and not carrying treasure, "might only have a 10% chance of drowning" (X51). A more concrete, more elegant solution, I've shamelessly borrowed from an old-school enthusiast and fellow Referee and will paraphrase as follows:

Swimming
A character traversing a liquid body the depth of which exceeds a logical depth for them to wade, typically six to seven feet for an average human, they must swim to traverse it. In so doing, the character rolls 1d10, modified by their Strength ability modifier, as appropriate. A roll equal to or under the Armor Class of the swimming character constitutes a success; a roll exceeding the Armor Class of the swimming character constitutes a failure.

• Succeeding on a swim check allows the character to either tread water - that is, remain in place and in control - or move up to half their normal movement speed for the round.
• Failing on a swim check means the character has gone under, sinking a number of feet equal to the margin of failure.

A check is necessary every turn to remain above water.

A Referee is encouraged to apply optional modifiers to this roll; added granularity and its associated complexity to taste:

• A character with no free hand - that is, holding or wielding an item in both hands, suffers a -1 penalty to the roll.
• A character with a wooden shield may - at Referee discretion - remove the shield from the arm and use as a flotation device, conferring a +1 bonus. A shield still bound to the arm, or a metal shield, contributes to Armor Class as normal for the purposes of a swim check.
• A character that has gone underwater involuntarily must roll a Save vs Spells or suffer a -1 penalty to further rolls, representing the panic of drowning.
• Particularly choppy waters may impose a -1 penalty; or particularly calm waters may grant a +1 bonus.
• A character's Dexterity modifier, if positive, does not count towards Armor Class for the purposes of swimming; conversely, a character's Dexterity modifier, if negative, should impose a proportional penalty.
• Magical bonuses - that is, the +1 from a +1 suit of armor or the +1 from a Ring of Protection - to Armor Class do not contribute to the target number of the swimming check.
• A character in metal armor suffers a further -1 penalty to the roll.
• A character encumbered by equipment or treasure suffers a penalty to the roll proportional to their level of encumbrance; bounded between -0 and -3.

Note that a "+1 bonus" or a "-1 penalty" in the context of roll under imply moving the die roll in the direction that would be beneficial or detrimental to the roll, as always. Thus, a character in Leather armor rolling an 8 would fail, the AC of Leather being 7; however, if the same character were to have a 14 Strength, thus conferring a +1 bonus, the roll of 8 would be considered a 7 (or, to keep with the addition motif, the Leather 7 would be considered a target number of 8) - meaning that their check would succeed.

If you, oh elegant rule-writer, are reading this and distraught by my hideous misremembering of your rule, please feel free to correct it in the comments with some of the vagaries I remember discussing with you in months past, but have - to your reckoning - misremembered.

Hats off, though, for the atypically usable contribution in the world of anonymous game discussion!

Public domain artwork pilfered from OldBookIllustrations.com on 9/6/2020 and cropped for use. Attribution in alt text.