I believe there is WAM that has no place on the UMD and that there is content on the UMD that has no place in WAM. For an example of the former, imagine that there were pictures of my first sexy romp in the mud with a girl -- an act that occurred when I was sixteen and my playmate was slightly younger. I have no doubt that, at least for me, that was a WAM experience. Yet any imagery featuring either of us at that tender age could, if a G-man squints just right, be categorized as outside the bounds of a law that agencies are particularly zealous when enforcing and courts are particularly harsh when issuing punishment for violations.
Some WAM, because of other elements in a given scenario, simply has no place at a venue that openly embraces the erotic dimension of the phenomenon. This may be true even if the scenario itself had an erotic context (though it is also true when the scenario has no erotic context whatsoever, like much of the wet and messy media produced by Nickelodeon.) On the flipside, we have the off-topic forum, the forum rules and other instructional writing, et al. that qualify as content without actually containing any sort of WAM. It may be that there is little more to this community than our common interest, but that does not mean there is nothing more to this community than our common interest.
All that said, I believe this is also the most eminent venue to sort out what WAM actually is as a fetish. I believe it is correct to exclude urine, vomit, blood, fecal material, etc. from WAM. Not only to those sorts of things tend to generate their own categories, but I suspect as our understanding of the human mind becomes more refined, we will discover that WAM has a much different psychopathology than watersports or scat fetishism. On the other hand, perhaps my personal inability to empathize strongly with people who possess those desires is causing me to miss something, and the only real difference between WAMmers and scat fiends is a function of aversion to a particular health risk and/or displeasing odor.
The main thing to remember is that each person is a unique individual. When conversing about sexuality (or filling out a kinky questionnaire) the category can be useful so that others understand "I'm into getting messy, but lets try to avoid significant vectors for human disease, okay." However, with an actual playmate, it is probably for the best to articulate desires and limits more precisely. Just imagine how enraged some members of the community would be if they met a beautiful woman who was into pie play, she got all dressed up (or undressed) just the way her new partner likes, then proved herself to be just the most horrible human being ever by going through the entire pie supply with hardly any facial coverage at all!

That is only one of theoretically infinite problems that could be averted by a dialogue that enables partners to understand in detail what areas of mutual delight exist and what sort of conduct is outside the bounds of what an individual could find exciting.
As far as "WAM" the term goes, my sense of it holds that actual bodily fluids are outside the bounds. Yet this is not an absolute and fanatical limit. For example, a spit take or even slurping up food and spitting it onto a partner may still be WAM, because the act is not about the saliva but rather about the food or beverage involved. People with a particular aversion to even trace amounts of the stuff have a duty to make that known, not only because of this narrow realm where it might become peripherally involved in WAM, but also because normal couples sometimes kiss in ways that are not absolutely and completely dry. As long as materials have not actually been swallowed (thus technically becoming vomit on the return trip, possibly accompanied by more bodily fluids than miniscule amounts of saliva) I believe they are still WAM-suitable. Likewise, stage blood made from food products, fake semen made from shampoo ingredients, etc. seem to fall inside the bounds of WAM even though they may deliberately resemble stuff that falls outside those bounds.
Ultimately drawing these fine lines can get tricky. For example, at what point does decomposing dung cross the line between shit and mud? Where do spoiled food and dumpster diving fit in, considering that these may also pose health risks and involve smells more wretched than edible food or comparatively harmless mud? What about industrial chemicals? Does the carcinogenic quality of used motor oil take it out of WAM territory? What if a substance is mildly corrosive? I think we could all agree that the kind of acid that burns on contact would not be part of WAM, but what about a paste that is only harmful if it is not washed away within a full day of application? We're all going to find personal variation in our preferences and limits, but the edges of WAM as a category are hard for any individual to pin down precisely.
My usual explanation is something like, "I enjoy seeing beautiful women covered in slippery, slimy, and/or sticky substances like food, paint, and mud," then add clarity as warranted by context. Wet clothing also excites me, but I actually cannot recall a girlfriend or other playmate unwilling to either join me for a walk in the rain or step into a shower while still wearing her street clothes. Usually it is the messy stuff that requires some discussion in order to build understanding. Of course, spelling everything out in cold clinical language is generally not the best way to go. Given a good rapport, learning by doing can work wonders. However, to the extent that your preferences might not be easily inferred -- that is the stuff you should convey with words in a timely manner. For example, I enjoy fully clothed women and totally nude women, but I find lingerie and swimwear to combine the worst of both. Since it is normal to find skimpy attire exciting, this is the kind of counterintuitive quirk I do well to explain with words.
Regards,
messydom