Maybe it’s because of all the cold weather, but I’ve really been feeling the tropical aquatic adventure thing right now. Unfortunately, the official D&D 5e rules don’t offer too much on that front. There are a few basics that are covered: characters making melee attacks without using a thrusting weapon such as a dagger, javelin, short sword, spear or trident get disadvantage. Ranged attacks are limited to their normal range and also have disadvantage, unless using a crossbow, net or a thrown weapon such as a javelin, trident or spear (although I would also nix the throwing axe). Also, creatures immersed in water have fire resistance. That’s about it—not a whole lot to go on…
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That might be enough for the occasional over-board incident during a fight with a kraken, but for an extended underwater stay there are a few other things that need to be addressed and some things that can really add to the overall atmosphere. You really want the underwater scenes to feel different than land-based adventures—otherwise, what’s the point?
If you have a system set up for handling airborne combat, that method can be converted for underwater use, with either the surface or the ocean floor as the reference level. Keeping track of each character’s specific depth would likely be way too over-complicated, so I recommend using a level system and using tokens or scrap paper to mark each miniature’s current level. For characters that are directly over one another, you can stack tokens or dice, or just have them share the space.
For a more advanced system, you can go the 3-tiered chess route and have separate grids that are marked with letters and numbers to track location (ie. Character 1 is on grid level 1, space D3 and Character 2 is directly above them on grid level 2, space D3). You can count horizontal and vertical squares as normal for calculating the distance of movement and ranged attacks, but area effects would take a 3-dimensional shape. A fireball will affect a sphere instead of just a circular area and a cone of cold would actually be a cone.
The DM might want to limit what spells and abilities are even possible underwater. Of course, you also have to decide what method they are using to breath and what light sources they have available. Quick and easy breathing enchantments are certainly one way to go, but it might present more of a challenge to do something like adaptive gills that allow no speech (or verbal spell components), or even a dive suit with a link to the surface or an air bubble that could be popped.
As for location, there’s a host of ideas that can make an underwater adventure unique. The PCs could set up base in a merfolk or sahuagin city made of coral, a tavern within an old sunken ship, or a storm giant’s underwater castle. They could delve down into canyons and caves inhabited by bio-luminescent creatures or slithering hunters in the dark. Personally, there are few things more terrifying to me than to find oneself over a drop-off, looking into a void of empty ocean that seems to swallow the light and threaten glimpses of some monstrous leviathan hunting for prey to devour. Anyone that’s played Subnautica can testify to how terrifying it is to hear that echo-locating roar reverberate out of the murk and know it’s hunting you (which might make a nice sound-clip for your players).
Things like currents, whirlpools, or even swarms of jelly fish can provide for difficult terrain. Some types of coral also cause burning pain with contact, so there’s plenty that can keep the players on their toes, even in an open area. There could be kelp forests inhabited by sirens or otyugh, underwater volcanoes spewing elementals, ship graveyards full of ghosts, or the empty shell of some ancient gargantuan sea-creature, now inhabited by who knows what.
That “who knows what” inhabiting the depths is also going to require some prep-work, though, as 5e is generally lacking on aquatic creatures compared to other areas. There’s variety that can be added to a general type such as merfolk, sahuagin, or naga, like coral armor wearing shark riders and seashell adorned shamans. Beyond the basic sea creatures and water elementals, there’s also no reason you can’t add other creatures and NPCs that have acquired waterbreathing the same as the PCs.
One of my favorite is a tribe of orca-riding orcs with waterbreathing war-paint that capture prisoners to work the diamond mines of their volcanic island fortress. Aberrations and things like reptiles that can hold their breath—*cough* dragons *cough*—is also a good option.
One final thing to note is the use of heavy armor; sinking like a rock might actually be helpful in some circumstances, but otherwise these players may want to look into other options. I like providing special enchanted seashell-armor that provides something like AC 16, as well as a decent swim speed.
Being forced to switch up gear, spells, and even companions can make an underwater adventure feel like a real expedition into an alien world. Getting players to think differently to overcome a challenge is always a good way to encourage player growth. As a DM, I would even have the players help brainstorm how the new environment would affect their abilities and what equipment they want to bring along. If done right, with a little bit of prep work, an aquatic adventure can be a unique and rewarding experience for the entire group.
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