I confess: the fear of water generally baffles me a little bit. All too often, folks bathe, wash their hands, and endure theme park rain, but getting splashed on Splash Mtn bothers them. I tend to think it has more to do with control than actual water. People dislike things they don't control. If so, empowering her with a plastic poncho could help.
I similarly find it a bit odd how many folks feel they must use paper towels to dry their hands when it is 90+ degrees at WDW. Give a little shake, and they will be dry inside of 30 seconds. We like habits I guess.
That said, it is wise to avoid wearing denim/heavy fabrics, some sheer fabrics, and suede footwear when riding a water ride. Quick dry fabrics and water tolerant shoes are both good ideas.
Also, one person's idea of "soaked," is sometimes another person's "just a few sprinkles." If OP's spouse doesn't like getting wet, then she's may lean towards thinking any water is too much. I don't think nayone is actually measuring the amount of water that hits them, ya know?
Where Splash is concerned, much depends on the water cannons. From what I have seen, they don't tend to turn them on when temps are low, and they don't always turn them on when temps are high. If it matters, you can watch the area from the bridge to see if they are running just before you get in line.
Also, this isn't 100%, but I think they tend to run the water cannons (more often) during the heat of the afternoon, and less often late at night/early in the AM. If you are thinking about booking a FP, consider an early/late FP time over an afternoon FP.
Last, well, I'll just toss this out there....if someone doesn't want to ride a ride, they will look for reasons not to ride it, and what they say might not be the only reason they don't want to ride it.