There are two that stand out for me.
1. The first Robo Cop has a very quick scene about 15 into it, where a beautiful woman has a cake smashed on her chest.
2. Back in the mid 80's, John Candy was in a movie with Bill Murray. The guys are in basic training, and one night they go to a club, and John Candy gets into a mud wrestling match with a group of women.

The clay pit scene in McClintock takes some beating.

Stripes. A classic!

I searched long and hard to see if that Robocop show was a real thing - alas it was just a parody created for the movie by Paul Verhoeven.

https://www.youtube.com/watch?v=g_cpGJnrLNM

(Caking at 0:17)

The movie "Eden Lake" is one of my favorites. I don't care much for the plot, but I've "watched" it about 10 times, fast-forwarding until I get to the wet and messy parts (which I watch at normal speed, but don't even bother to put on the sound for).

Highlights: Towards the beginning, a woman is thrown into a lake (wearing a nice, long white dress). As the movie progresses, she is chased through the forest and the dress becomes progressively dirtier. My favorite part is when she hides in a dumpster and emerges completely covered in muck.

As in these stills.

Eden Lake is very good but my all-time favourites are The Silencers, Sweet Movie and Tommy which have all been mentioned before.

Alan

I've actually been looking for this one for a few years. I stumbled across it on YouTube several years ago, but couldn't remember what it was. Loving the choices here!

Two more that came to mind. Both are the final scene before the credits (save the best for last, right?), so I'll be economical with the details.

Hackers: Not messy, but wet. Angelina Jolie and a pool. Need I say more?

Antiporno: This one involves a Japanese schoolgirl, cake, and paint.

I always love the parts from Doctor Quinn where Jane Seymour is in the water in her clothes, the mud scene from Relic Hunter, and the mud scene from Django.

Another one I recently thought of. Alison Lohman in the finale of Drag Me To Hell. She's dug her way into a grave in the middle of a downpour, and she's getting soaked and muddy. Then at the end when she emerges from the grave they do a superimposed thing that goes from her being muddy and wet to being in a shower getting cleaned.

I haven't seen the movie, but judging by the trailer, the newest Jurassic World appears to have a scene where a redhead jumps in a lake (around the 1:30 mark https://www.jurassicworld.com/videos/). So that should be fun.

Yep! I'm actually about to see it within the next 2 hours, so I'll confirm whatever WAM scenes feature afterward.

EDIT: Unfortunately the aforementioned wetlook scene is the only instance of WAM in the film, and short-lived at that.