Duo can sort of swim? I've never headcanon'd this for him. He grew up on a poor space colony, so I'm not sure what kind of swimming access he'd have. It's not explained at all in canon how he turned up on a dock early after setting his Gundam on autopilot to pull Wing out of the water when he'd been under water in the cockpit with it. So, yeah. That could be fun exploration there, and I'd be willing to plot something with someone if they want to rescue or be rescued. Something tells me he could probably end up in either situation.
Kitty knows how to swim. She's probably not an amazingly strong swimmer, so she might not be great at staying afloat in flash flood conditions or at rescuing people. But, I'm definitely up for having her rescued by someone! Does a strong swimmer wanna help her out? She can also probably phase people for a time, but she and her "passengers" would need to be able to hold their breath, so there's that limitation on it. Let me know if you're interested in anything.
Piper can swim, and doesn't fear water quite as much as her older sister did, so she'll be okay. The idea of drowning, though, is a small trigger with the Halliwell girls because that's how their mother died, but it's mostly Prue that's affected by it because she saw it happen. She'll probably get more upset as things get more dire looking. But, she can cast spells and might be able to freeze some potential washouts early on, so if anyone wants to plot out early rescues or keeping her calm later, I'm up for plotting!
Tieria can probably swim. And, even if he hasn't before, he can figure it out really fast. He also has the ability to hold his breath for really long, and has a high endurance for long term treading water. He could probably help some people out if needed. I'm open to ideas with him!