By Alan McCoy from Dungeons & Dragons: Fundamentals
How to build challenging encounters against a Water Elemental
Note: This has been prepared utilizing only the 5E Core Rules. The Water Elemental can be found in the Monster Manual page 125.
Water Elementals are summoned, not evolved creatures, as such they have no specific form, they can be bound into a shape by the will of their summoner or creator. When speaking of Elementals, these terms can be used interchangeably. When summoned without a specific shape requirement, they tend to take a shape that fits their nature such appearing as a cresting wave that rolls across the ground, perhaps with the vague outline of a face. A water elemental in a large body of water is nearly invisible.
A Water Elemental cannot be summoned to an area unless there is a significant volume of water or at least water based liquid present.
Step 1) Let’s review what we know about the Water Elemental
Water Elementals have extremely high strength and toughness, they are highly nimble. They have very little in the way of personality and intelligence.
Water Elementals swim amazingly fast (90 feet/round) they move across the ground at a far more normal 30 feet/round. As they flow rather than walk, some types of difficult terrain are no problem for them, others might be (DM determination) They can move through an opening a mere one inch in diameter without slowing. They can also enter a hostile creature’s space and stop there.
Water Elementals take half damage from acid and non-magical weapon attacks (bludgeoning, piercing and slashing). They are also completely immune to poison.
Water Elementals are conditionally immune to Exhaustion, Grappled, Paralyzed, Petrified, Poisoned, Prone, Restrained and Unconscious. They do not need to consume (eat or drink), breathe or sleep.
Water Elementals can use their Whelm ability to form a wave which it uses to overwhelm grapple and drown targets. The Water Elemental is a size large creature and the wave can affect multiple enemies.
Water Elemental senses include Darkvision 60 feet, they have a passive perception of 10.
Water Elementals are not vulnerable to cold damage, but the cold does slow them until the end of its next turn.
Step 2) Determine the probable Strategy
Water Elementals are constructs that exist only to serve their creator. They do come with certain default combat techniques built in that serve much like the instincts of an animal.
Their low intelligence makes it hard for them to change tactics, thus they will often continue even a losing strategy unless directed to do something else by their creator.
These elementals maintain a telepathic link to their creator if that creator remains within 120 feet. The creator can receive the sensory input of the elemental.
Step 3) Determine Tactics
The primary attack is of course their multi-attack slam, using the water and or available debris to bash their target into submission.
With their Whelm ability the Air Elemental can enter the space of a hostile creature and restrain them, pulling them under to drown the target.
The Elemental can grapple one large creature or two medium or smaller creature at a time with this ability. Targets caught with this ability are considered restrained.
Restrained targets cannot move. Make attacks with disadvantage and are attacked with advantage. They also have disadvantage on dexterity saving throws.
Additionally, these powerful creatures have a few programed techniques available to them.
Technique #1: The Shove – The Elemental can sacrifice one of its slam attacks to push an opponent back five feet. It will do this if the target is near the edge of a drop-off or otherwise could be endangered. In a large body of water, the Elemental will push the target under.
Technique #2: Critters – The Elemental can pull up dangerous animals from the depths providing access for them to attack.
Technique #3: Washout – The Elemental can swim around pilings or other dock areas to loosen supports and cause their collapse.
Technique #4: Capsize – The Elemental can direct it’s Whelm Attack against the side of a smaller vessel and overturn it. Larger vessels could find their hull battered and eventually holed by the powerful slams.
Technique #5: Rudderless – The Elemental can surround a ships rudder, steering it to doom despite the crew’s best efforts to change course.