- Every plane has a primordial on it, not just so-called Elemental planes.
- The Prime Material Plane's primordials are not humans or elves.
Having said that, let's look at what we need for a creature composed mostly of water or a liquid-substance-similar-to-water. To keep its shape on the Prime Material, the air pressure here must be higher than the atmospheric/water pressure in its home plane. The higher pressure on the Prime Material plane would help the Water Primordial to have a defined shape while traveling here. The ability to keep their shape here would make the Plane of Water Primordials more like a world of mist instead of a vast limitless ocean.
For the Water Primordial to move comfortably while here, the creature must be incredibly strong. I say that because the water elemental can move close to normal human rate.
Let's say that this Water Primordial is more like an amoeba instead of a cohesive body of water. I imagine that this means that they eat by absorbing its victims into its body. Taking advantage of their great strength, we can say that they might crush their prey (like a boa constrictor) before absorbing it into their bodies for digestion.
To make them more gruesome, maybe the water primordial eats like some species of starfish. They vomit out their digestive organs into the victim and digest the victim from the inside-out.
Odd that the Fire Primordial is a tea-drinking vegetarian and the Water Primordial is a carnivore. For what it is worth, I used to be a tea-drinking vegetarian, then a tea-drinking omnivore for many years before becoming a non-tea drinking omnivore.
Being translucent, seeing a Water Primordial is bound to be a harrowing experience. In fact, I imagine that they would be confused for undead quite often while they are here. Taking advantage of this, one of the strategies Water Primordials have devised in order to survive is a fake undead attack. Those that have fed more recently walk slowly in front of the others yet to feed. While intelligent prey re-double their efforts to deal with a perceived undead attack, the hungrier Water Primordials can get close enough to attack.
Water Primordials suffer great damage from contact with salt. In fact, their extreme reaction to Fire Primordials is mostly due to the sodium chlorate in their system. However, sentient species on the Prime Material have found other things to fend off would-be Water Primordials. The most common is regular table salt. Water Primordials can swim in fresh water, but cannot survive short visits into an ocean. Other substances found include a reddish mineral found former seabeds that have dried up and certain kinds of weathered volcanic rock. However, the most effective defense utilizes a common foodstuff, dried rice. As little as one cup of rice is enough to make a Water Primordial reconsider an attack.
Again, it's important to me to find an everyday material that would allow normal people some meager protection against any powerful creature. I come from a world-building perspective that the craftiness and intelligence of a sentient species should lead to innovative defenses that do not necessarily rely on magic. Think about it, if a high-level mage creates a super-weapon or mysterious substance to wipe out a certain menace, the town leaders or villagers are beholden upon the mage to continue to protect them. Humans generally cannot live with a situation like that for long periods of time, especially if they are paying for that protection.
When it comes to a rival race on the Plane of Water Primordials, I imagine a race of creatures that live on the various floating islands on the plane. They would use stone and leather weaponry and armor in order to avoid the expense of dealing with rust. These weapons would not be slashing or piercing weapons, but hammers. I want to give them a distance weapon, shaped like a round plate, that when thrown, lands flat on a target. In other words, instead of disc slicing through, it would be more like a belly-flop in a pool. The side with the largest surface area strikes the target.
I would imagine these rivals would eat the exotic floating/flying fish native to the plane. I also imagine that they could cultivate some plants and fruit bearing trees in a limited quantity.
So now, another poll about the name of The Plane of the Water Primordials.