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I have a potential scenario coming up where my PCs will be in the sewers under a large city. There will be a Circle of the Moon druid who likes to Wildshape as soon as the party enters anywhere dungeonesque. Since there is going to be water in the sewer they might want to use the chance to shift into a giant octopus (or other aquatic shape).

My Question is: How deep would the water in the sewer need to be to allow the player to use the 60ft swimming speed of the octopus over its 10ft walking?

I'd like to accommodate the use of different wild shapes whilst still having the sewers to be realistic.

I'm mostly looking for specific rulings in RAW, or in the rules as intended by the authors, to ensure fairness to the players and allow for parts where they can and can't swim/breathe.

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    \$\begingroup\$ I think the real concern is that you want to swim in the sewers. That's just straight up poop water, yo. \$\endgroup\$ – Southpaw Hare Dec 6 '17 at 20:20
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In D&D 5e, a Giant Octopus (MM pg 326) is a large creature taking a space of 10 feet by 10 feet (MM pg 6) or a 2 by 2 square. In both the DMG and PHB (DMG pg 116-117 PHB pg 182/198) nothing is said about how much water is needed to allow something to use its swimming speed.

My personal ruling, in the case of a Giant octopus, I would require at least a 5 ft depth (half the height of its space) to allow it use its swimming speed, but let the player know that any aquatic form they transform into may need a greater depth than half the height of its space in order to perform certain actions (Like a Great Reef Shark jumping out of a river to bite someone on a bridge vs Great Reef Shark jumping out of an ocean to bite someone on a canoe.)

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  • \$\begingroup\$ Would it be appropriate to rule that if you don't have at least your space in water depth and want to use your swim speed, you're Squeezing? \$\endgroup\$ – Darth Pseudonym Dec 8 '17 at 21:54
  • \$\begingroup\$ @Darth Pseudonym In D&D 5E the condition "Squeezing" does not exist at least in the PHB and I have never come across that condition. Unfortunately, I don't see any condition that could be used as a substitute. I made sure to word my answer "In the case of a Giant Octopus" because my ruling of at least half the height of its space may not make sense to apply to all aquatic animals (Example: Killer Whale). \$\endgroup\$ – KDodge Dec 9 '17 at 3:51
  • \$\begingroup\$ Also, Squeezing in my mind would be like a Medium creature trying to fit into a Small sized door, not really applying to swimming in shallow water. I can understand where you are coming from, but I don't see how that would make sense. Again, different aquatic animals would get different rulings for the sake of making sense. \$\endgroup\$ – KDodge Dec 9 '17 at 3:51
  • \$\begingroup\$ Player's Handbook, page 192, first header: Squeezing into a smaller space. "A creature can squeeze through a space that is large enough for a creature one size smaller than it... While squeezing through a space, a creature must spend 1 extra foot for every foot it moves there, and it has disadvantage on attack rolls and Dexterity saving throws. Attack rolls against the creature have advantage while it's in the smaller space." \$\endgroup\$ – Darth Pseudonym Dec 10 '17 at 15:59
  • \$\begingroup\$ Squeezing applies any time you try to move through a space that's too small for your space, regardless of what makes it smaller. A large creature's space is 10x10, so if the water is less than 10 feet deep, it makes sense to have the monster treated as squeezing if it wants to swim, or it can go with regular movement and not be treated as squeezing. \$\endgroup\$ – Darth Pseudonym Dec 10 '17 at 16:08
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RAW there aren't any. DM call.

Realistically, it's going to vary depending on creature -- a killer whale will need much deeper water than an octopus, even if they are the same size.

The octopus is a great choice for this scenario, as they are extremely flexible. I would rule that a giant octopus could swim in as little as 3' of water, based on having observed octopodes several times.

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If you turn into a giant octopus then I think the whole creature must (theoretically) fit into the space without touching any surface.

Note that if the octopus wants to go upwards it will probably be better to climb up, instead of trying to swim especially in tight spaces. And also that octopi can fit through very tight spaces, since they have no bones (one bone+beak)

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Buoyancy or lack thereof is the reason why an octopus is slow on land (as are walruses, stranded whales, ... ).

So the amount of force going into supporting versus propelling itself is what slows it down, and that scales pretty much with how much of it sticks out of the water.

Thus I'd rule: If it's half-under, that's 10 + (60-10)times(1/2) = 35ft, if three-quarters under you have 10 + 50 times (3/4) = 48.5ft; so as a formula 10ft + a times 50ft with a the fraction under water (0 to 1). So this depends on the diameter of the octopus in swimming motion, i.e., all arms more or less together, widening up for the next push; not arms-wide 'crawling over seabottom' type motion (which I'm interpreting the "10x10ft" of PHB as). Look at videos of escaping octopuses.

Of course, you can subtract further speed penalties for obstructions (a la 'difficult terrain'): A narrow bit, however deep, will slow it down; dumped bicycles/supermarket shopping cart littering the channel, a 'fatberg' (yeah, google some horrific London videos!), ...

Note: Plural of octopus or walrus (and cactus) is octopuses or walruses (and cactuses) --- we're speaking English here, so (pseudo-)Latin plurals (pluri?) are irrelephant; thus no octopi, walri nor cacti please.

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  • \$\begingroup\$ I suggest that instead of recommending that algorithm (since it is a bit complex as far as game algorithms go), you recommend a simple algorithm of "a * speed, minimum 10". It lets us just straightforwardly half or quarter or dump it down to 10 in most situations. \$\endgroup\$ – doppelgreener Dec 6 '17 at 16:48
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    \$\begingroup\$ Also octopus is greek-derived not latin-derived so the pedantically correct plural is octopodes. :) \$\endgroup\$ – doppelgreener Dec 6 '17 at 16:49
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    \$\begingroup\$ Buoyancy is absolutely not the reason why marine animals are slow on land. They're slow because their propulsion mechanisms are evolved to work well in water, not on land. This is simply a result of the fact that the morphology of fins and water jets just don't work well on land. When submerged, only apparent weight changes, not mass, so the same force is required to move laterally. \$\endgroup\$ – Nuclear Wang Dec 6 '17 at 18:38

protected by doppelgreener Dec 6 '17 at 19:39

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