I believe water coming through blocks is a "yes/ no" decision, one not requiring more power than the other. A tiny bit more processing power is dedicated to flowing *further*, and which way to do so. But it won't be any more processing power than dumping a bucket of water on some random spot on the uneven ground.
People interested in water behavior and how much power it takes could use the Carpet Mod (showcased by gnembon in this video).
I would suggest setting up different scenarios using running water and then the carpet mod to determine how much processing power is actually used between the *start* of dumping a bucket of water vs. water that has already completed running down a slope and runs no further. See if you can figure out if there is a difference in processing time used in "running" and "completed". As a *guess*, I wouldn't think having a "fence" in the way would matter vs. nothing being in the way, but if you set up a good test environment we can determine if my guess is accurate or not when we have a snapshot with this feature, presuming a Carpet Mod version would be quickly enough made for the snapshot.
I also want to voice appreciation to @Vampire_Asunder. I also don't get the interest in expressing, on one hand, "I don't like anything since Minecraft 1.four_or_five_years_ago" and tying it with, "so I hate this new feature and I wish Mojang would focus on what players really want."
People are *STILL* doing NEW game development for the Commodore 64 computer. See here: https://retro.moe/2015/02/01/coding-for-the-commodore-64-what-happened-in-the-last-25-years/
And that is OK-FINE. I really respect Grian, and he's playing on a server with a minecraft version from before they had hunger bars. And that's cool. If you like that play, you *SHOULD* be doing that. Heck, want to be really cool? Do a series of videos promoting the differences between the *current* version and Minecraft 1.four_or_five_years_ago, and showcase the fun to be had on that version, and *specifics* on how Mojang could bring the fun *BACK* by returning those features to the current game. That would be useful and constructive for the community. "Every new version since 1.four_or_five_years_ago sucks so I hate this new feature and you should too..." Not so constructive. The game has moved on, WHILE ALLOWING PEOPLE WHO WISH TO ENJOY OLD VERSIONS. That's way cool. Very few games allow anything like that. If you want to stay back, do so, and enjoy. But Minecraft as a game has moved on. The bulk of the community has moved with it. I have attempted to build things underwater, and I've also enjoyed underwater builds from maps by some really good content creators. Underwater "coral" and "seaweed", sea wrecks, and so on. The "underwater air pockets" are horribly annoying. The Vanilla ocean floor is lifeless and dull. In my opinion, this change is necessary and a long time coming. I'm excited about it, but I'm open to constructive ideas on how this can work without harming current common underwater structures (such as with farms) or ways we play.
But "Nah, I only play 1.whatever, the best Minecraft version ever, and this water physics thing suxxors" won't really win me over.