I always thought it would be a cool ability that could fall into our domain - basically allows swim rooms (rivers and whatnot) act like standard movement rooms for a limited time (likely a commune addition but since I don't know which/where to put this sort of thing I put it in here)

I can swim all the rivers at this point - and don't ever encounter any Athletics checks (concerning water) that give me much difficulty but the time saved during travel would still be beneficial so this sort of ability would have some staying power from the moment you get it. As opposed to some of our abilities that seem become somewhat mundane/useless over time (See my posts on why we should make adjustments/improvements on the Align ability (an awesome ability that eventually becomes pointless)

Oooh, I like this idea. If Eluned's Second were changed to this, I could be convinced to collect fish guts for it. (Barf.)

For balancing mechanics: walking across water with the commune active could still check if you're able to swim it. If you'd fail the check, you end up in the water, flailing around again. With no RT, the successful walk shouldn't grant experience, either. I'd be okay with that trade-in for how ridiculously awesome it'd be to literally tread water.

Thread rez! Water walking would be neat and brooms/carpets already exist that bypass swimming entirely so the mechanics are definitely there.