What would happen if a character were to attempt to walk on the waves of a very tumultuous sea, using a Ring of Water Walking?
Would they automatically remain standing on the waves?
Or would these waves become something similar to a solid ground during a severe earthquake, requiring the character to perform dexterity saves to remain standing or fall prone?
As usual, there's no single clear answer to anything that isn't explicitly stated in the rules. A DM could certainly decide that waves represent an uncertain surface that the PCs will have to make rolls to move across; but they could easily rule the other way, since it's magic that says you can move across water 'as if it were solid ground', and solid ground is not generally known for heaving up and down under your feet. The latter interpretation does have some basis in our world; the original Biblical example of walking on water, which presumably inspired the spell and ring, took place in a serious storm with large waves.
In general, I think the real answer comes from answering the deeper question, "What do you want to accomplish by calling for rolls?"
If there's a fight or other challenge happening and you, as the DM, want the waves to count as an environmental problem that impacts the PCs but not their aquatic enemies (thus increasing the difficulty level of the encounter), then I think that makes a pretty great fantastical setting for the scenario.
By contrast, if you're considering just having the PCs roll some checks to cross the stormy area, but those checks don't come with any actual consequences for failure (usually taking a longer time to cross an area and looking like an idiot while doing it aren't actually consequences), probably just skip it and move on to the next point of interest. You can describe them stumbling and sliding across the waves if you want to have a comedy beat, I suppose, but calling for checks in this scenario sounds a lot like the classic newbie-DM mistake of having the players make tons and tons of inconsequential rolls.
It's also worth asking yourself if your plan is eliminating the benefit of the magic item in question. If the ring is allowing the player to walk on water, but you're making them functionally perform the same rolls you'd call for from a swimming character, then you're kind of taking away the coolness and benefit of having a magic item that's perfectly suited to this challenge, and that's usually a bad thing.
While the ring clearly says you can walk on water, it is very unclear as to what happens when that water terrain becomes problematic. This gives the DM and the table a lot of leeway as to what they want to happen in these cases.
However, with leeway also comes a requirement to set expectations for your players. I was playing at a table where we had to traverse a few hundred yards of stormy seas to a dock after our ship was destroyed. I had a ring of water walk, but it was very unclear as to how it might work in this situation.
I ended up having to go with the flow of the DM, who basically hand waved the ring's abilities away and I was treated the same as the other players within the environment. I think I would have been better with that decision had it been made clear ahead of time so that I expected it to 'not work'.
But at the end of the day, the encounter is about the rough seas. How you want to envision that encounter is what really matters. Limitations, requirements, and the game itself are all here to tell a story together. As long as your mechanics are clear, consistent, and fun, then whatever you choose to do in this case will work.
Without a clear answer, the only answer remaining is the one that brings the most fun to the table.
The ring grants the ability to stand as if water were firm ground.
In a severe storm, with large waves, I would treat this either as if they were surfing (with an extremely buoyant life vest that would return them to the surface should they have a wave break over them) or as if standing on an immense trampoline, depending on the exact situation and which would be more entertaining.
As a DM, under no circumstances would I simply rule the ring does nothing, or create some punitive outcome.
It's not your job to act as a foil. It's a situational item, let the player have fun with it when the chance to use it comes along.
I don't always invoke the Rule of Cool, but when I do, this is the sort of scenario it should be applied.
The critical part here is the word "can" in the item's description. The Water Walk spell on which the ring's magic is based grants the ability to walk on water as if it were surface... abilities are used when you want to use them, not always. Agency is important in D&D.
So the wave of water will act as liquid unless the wearer intends to use the ability on the wave (which would result in the wave pushing them back and, if a DM chose, they could possibly cause fall damage if they wanted to (same as being thrown hard against a stone wall, for example)).
One comical scenario of a use would be if the wearer were to try and jump and catch the crest of the wave in order to climb over it, but that would be difficult given water's fluid state (and a wave's constant motion).
This is just my opinion and the DM would have to be the one to decide but, I would say the water would be difficult terrain and going up a high wave that is going almost diagonal would require you to climb. I hope this helps! Tell me what you think!
