Hi,
Firstly, can I just say that it's fantastic that you are looking into this and wanting to understand. As somebody who suffers from SPD it is wonderful to have family members care enough to want to try to make sense of it, to help.
Secondly, *hugs for you*. I understand how scary that situation must have been for you, particularly in front of your child.
Have you looked at the resources about SPD on this page:
http://spdsupport.org/resources/index.shtml
The biggest thing that might help you understand what's happening with your wife/allow you to open a dialogue with her might be reading a book such as
Too Loud, Too Bright, Too Fast, Too Tight. It explains what is going on with SPD, the signs/symptoms, the differences between things like meltdown/overload/shutdown, and how you can do things to try to minimise how SPD impacts on your life. I found it incredibly useful to understanding myself and my mum is currently reading it.
I'm still new to this, so I'm sure others here will be able to be more helpful/clear. It sounds to me like your wife had a meltdown. And the frustrating thing for the person with SPD as well as other people around them is that what might one day be fine, because the other potentially stressful stimuli that day were minimal, the following day might not be if there are other things that have already bumped your 'arousal' level up a lot or its unexpected. For example, for me that might mean that driving 3.5 hours in the car with heavy rain for part of it and the windscreen wipers going on full might be fine if that's the only stressful stimuli I've had that day. If as well I have been dealing with 50-100 kids for 3 hours before that in my job, then it might not be okay and send me into overload.
Things to think about: with the water fight the other day who initiated it? Was it you or your wife? If she initiated it she would have been aware of what she is getting herself into and prepared for it. Often the unexpected is not good with SPD. What was she wearing? If it's the feeling of clothes with getting wet, then was she wearing less clothes for it to be an issue? Different material?
I guess one of the important things to realise is that when we head into things like meltdown/overload etc. it's often impossible to be able to easily pull yourself out of it/control it. We're actually pretty much not in control at all, the nervous system and fight/flight/freeze primitive responses have taken over. Things have been evaluated by that as bad or dangerous (even if they wouldn't be for the 'normal' person). For example, while this site talks about kids, it's pretty much the same things for adults:
dealing with SPD meltdowns. Sometimes it can be hard to know that you're heading for it - if you do, you might be able to use techniques to stop it building up to that bad level (if you have the techniques). Tiredness and stress generally add in to it.
Everybody has a different list of triggers - some people are hypersensitive to things, sometimes badly hypersensitive. Other people are sensory seekers and need that input or they might head into meltdown.
We can often feel like we're just weird, that people aren't going to understand, they'll make fun of it, or that we hate having people have to 'cater' to our issues, which might all be reasons why she's never mentioned it before.
Use this incident and her mentioning it to start a dialogue - read up and see whether she's willing to talk about what her particular form of SPD means in regards to how she interacts with the world (don't push too hard, though, if she doesn't want to talk about it to begin with, it might be hard for her to). Look for the things that might provoke nervous responses from her, or "I hate..." - think about whether they might have a sensory basis.
It may also be a case that your wife may need some more Occupational therapy etc. to help her manage things better - but probably only something to be brought up after she's happily conversing about what SPD is for her.
Have a look through some of the threads on her - there's threads that talk about what overload feels like to you, etc. that might give you some more insight. Ask more questions, we're here to listen and hopefully help.
*hugs you more* I hope this helps with your first step to understanding what's going on.