Underwater Adventures

Underwater Adventures describes those parts of a game campaign that unfold in the aquatic realms of oceans, seas and lakes, environments both strange and alluring for player characters to explore. Beneath the waves lie ancient submarinal races and civilisations, dwelling in green-lit realms populated by fish-like humanoids, ever under threat from predatory monsters. Stories of lost treasure beneath the surface often tempt players with the promise of wealth that rivals or surpasses anything found in surface dungeons, from pearls the size of grapefruits to sunken ships laden with coin, scattered across the ocean floor and waiting to be claimed by bold adventurers.

Yet the difficulties of exploring the ocean are as fearsome as the monsters that inhabit its depths. Surface-dwelling humanoids are not made to breathe water, nor can they swim with the ease of creatures born to the sea. Though the treasures below may be vast, the characters' limitations in such an alien environment are a major reason why so much wealth remains undisturbed. To manage underwater escapades, a set of special rules must be observed by the players. This page is intended to assist the DM in handling such adventures with practical guidance, offering both challenges and excitement for the players to engage with.

Breathing Underwater

A primary concern for the party is the ability to breathe underwater. This can be addressed in several ways. Useful spells include water breathing and airy water, while magical items such as a potion of water breathing, the helm of underwater action or the robe of the manta ray offer more enduring solutions.

Another approach involves the use of shape change or polymorph to assume a form better suited for breathing and maneuvering underwater. Certain herbs and sage abilities may also provide limited methods of underwater respiration. Caution must be taken with medicinal alterations, as such changes may grant temporary underwater breathing at the cost of losing the ability to breathe air normally during that time.

Visibility

Without special adjustments to the character's ability to see, such as the use of goggles, characters will find that water is far more opaque than air. Clear underwater vision is limited to 20 feet, or four combat hexes. Between 21 and 30 feet, all attacks suffer a -2 penalty to hit; this penalty increases to -5 between 31 and 40 feet. Beyond 40 feet, only objects larger than 10 feet in diameter can be faintly seen, up to a maximum of 60 feet. Beyond this, nothing can be discerned except a hazy aquatic background. These visual limitations may be completely overcome with the use of the ultravision spell.

This naturally places an adventuring party at a severe disadvantage, especially as many underwater monsters are capable of swift movement. Such creatures can emerge from the murky distance in moments, and if they win initiative, they may strike a character and slip back into the gloom before a response can be made.

Depth	Illumination	Reduction to Visibility
101-150 ft.	Dusk	-20%
151-230 ft.	Full moonlight	-30%
231-360 ft.	Dim moonlight	-50%
361-570 ft.	Starlight	-70%
below 570 ft.	Full dark	-100%

Worse, all visual range can be severely reduced by declining illumination, whether from increased depth or the natural loss of light at night. The table shown lists visibility reductions caused by either condition — and when both apply, their effects are cumulative. For example, if a character is 120 feet underwater during dusk, visibility is reduced by 40% of normal.

Illumination can counteract this decline, but any light source that relies on flame or fuel is useless underwater. Water absorbs light at a rate of 20% for every 10 feet from the source. This means a light spell is only 40% effective at 30 feet, and becomes ineffective beyond 50 feet. Comparing this to the table, a -50% visibility reduction would be fully canceled within 20 feet, reduced to -10% from 21 to 30 feet, then to -30% from 31 to 40 feet, and offer no benefit beyond that distance.

However, a character can observe an illuminated object in underwater darkness up to a distance of 120 feet, even when nothing else is visible in the surrounding murk. This offers a crucial point of reference, helping characters orient themselves and avoid becoming lost. Such locations should serve as safe refuges for the party, ideally positioned among rocks or within a small, defensible cave. No character wants to be caught in total darkness. Many dangerous predators are fully adapted to reduced illumination and can see without difficulty, leaving the character at a severe disadvantage.

Sound

While it's true that the suppression of voice and hearing among party members might enhance the underwater atmosphere — encouraging players at the table to rely on hand signals instead of speech — in practice, this quickly becomes frustrating and unmanageable. To avoid this, water breathing magic includes a feature that allows characters to speak with one another as they would on land.

Unlike sight, sound travels more effectively in water. As a result, magical adaptation enables characters to speak clearly to a distance of 120 feet, twice the usual range on the surface. Shouting can be heard up to 240 feet. Magic also enhances directional awareness of sound, allowing characters to identify where a noise is coming from. Though an enemy may be out of sight, it is still possible to detect their presence by sound alone. With vision limited but sound perception improved, the overall likelihood of surprise remains unchanged.

Movement

As ever, the character's movement is determined by the number of available action points (AP). Characters with no experience in swim are at a strong disadvantage, as their movement is reduced to stride-½, requiring 2 AP per combat hex. Characters with even 1 knowledge point in swimming can paddle at a speed of 1.2 hexes per AP, with more experienced swimmers capable of even greater speed. For most other AP-related actions, no adjustments are necessary.

Swimming while holding a weapon or any item in hand significantly hampers mobility. Any object gripped reduces forward movement by 60%, making it wiser to sheath items, move and then draw them again, rather than carrying them while in motion. This limitation leads to the classic underwater trope of carrying a small knife (1-3 damage) in one's teeth, as a dagger would be too large for such use.

In a submerged environment, assumptions about movement must be reconsidered. Depending on its properties, a dropped item may float upward instead of sinking, or it may move slowly enough in either direction to be retrieved shortly after. Uncorking a potion to drink will allow some water to enter the flask, but the narrow neck keeps this from affecting the potion's effect. By contrast, opening a scroll case underwater would almost certainly destroy the scroll inside.

Lateral & Vertical Movement

Just as the combat map is divided into 5-foot hexes, a three-dimensional combat map should use the same scale for verticality. This means a character may swim any number of hexes vertically or laterally, with both directions using the same basic measure. When combining vertical and lateral movement, trigonometry is used to calculate the true distance — with the understanding that a character must fully enter the intended hex.

For example, if Tasha wishes to swim two hexes upward and two hexes laterally, the total distance travelled is 2.82 hexes, or 14.14 feet. Tasha must be able to swim that far in one round to reach the destination hex. If her swimming limit is only 12 feet, she must instead swim two hexes laterally and one hex vertically, or the reverse. Fractions of hexes are ignored when the round ends.

Swimming upward or laterally costs the usual amount of AP. However, swimming downward costs twice as many AP. In the above example, swimming down two hexes is treated as four hexes of movement, not two. Swimming down two hexes and then two hexes laterally counts as six hexes of movement, even though the actual distance is only four — requiring the ability to swim 23 feet (4.47 hexes).

See also,
Free-diving (sage ability)
The Adventure