OP, I've had similar problems a couple of times.
One on an outside retaining wall (not connected to my home) when I lived in Arizona, but it was there to keep the water from the adjoining neighbor's property that was higher than mine, away from me. The problem is, as water hits the cinder block, it does soak through after a while, leaving a white residue on the other side. Looks hideous, but was not a super big deal. What was a bigger deal is as time went on, small cracks appeared in some of the blocks from settling... THEN, when the monsoons came and the neighbor's property essentially flooded, I had spraying water coming out from those places onto my yard, deck and side of the house. NOT GOOD. Yes, it was just like that little Dutch Boy and the dike, except picture me there. Boy oh boy. What I had to do was get some type (sorry, don't remember specific brand) of sealant designed for that purpose, effectively caulk the cracks, then apply a sort of rubberized membrane paint/glue stuff over my side of the cinder block. That kept the water away and I didn't have any more problems the following year.
My second situation is in my present garage. Cinder blocks on the lowest level that come up maybe 5" above the slab of my garage. Studs and drywall sit on top of that for the walls. The home is nearing 40 years old, and living in California, well, there were a lot of cracks when I bought the home. I had a structural engineer come out to look at another problem in the overhead beams (not related to the subject at hand) before I made an offer on the home and had him suggest what to do with the cinder blocks. He could not find cracks in the slab or cinder blocks that went all the way through and there was no water that had come down to those areas from up above. His recommendation was to get a wire brush, clean off the junk, use my ShopVac to clean out the cracks, then put some caulk designed for that purpose (I found it at Home Depot -- just read the labels and you'll figure out which one works for that purpose) in the cracks to prevent insects, etc getting in or through. I did all that, painted things to match, and have not had a problem since.
My third situation (and I started with only 2) may not be related, but just in case as this thread has gone off on roofs and gutters that may or may not apply to you. Also, in my present home, when I bought it, it had no gutters. A month after I moved in, we had torrential rains and I ended up with more than 6" of standing water on 2 sides of my home -- to the point it was within 1/2" of coming over my back slab step and into my sliding door into the family room. I effectively had a lake in my backyard against my home for nearly 3 days after the rain subsided. I was going to have to put a new roof on my home and fix a structural problem in my garage, so I went ahead and as that was being done, hired landscapers to dig and bury in drains along the 3 sides of my home. When the roof was done, I had a gutter company come out and put that up, and the downspouts connect into the underground drain system that flows under my front lawn and sidewalk. Now in a heavy rain, it can blast out two holes in the curb with water from my roof, but hey, my property is bone dry, with no more lake around my house. What I unfortunately found after that is the problem had likely been happening for years ... I had to pull my wood floor up in the whole living room/dining room that adjoined the wall that sometimes had the lake on the other side ... it was evident the water had absorbed into the stucco, through the walls and caused mold to form under the glued-down wooden floor. Well, I took care of the problem on the outside, but then had to replace the interior hardwood floor with all that other stuff that had to be resolved.
...so, all that to say, I'm not a structural engineer or really any kind of Mr. Fixit myself, but it seems what you're trying to do should work. Do see if you can determine where the water is coming from ... if it's from the other side, that's one thing, sort of like my first or third scenario. If it's coming from up above, I would think you'd see something in your exposed framing or maybe on the drywall... then you would really need to investigate the roof situation -- otherwise don't worry about the roof. You need to prevent the water from coming in wherever that is, as well as correct the inside of your garage because it seems you're like me and want things clean and tidy once again. ...but your beloved SLK should be fine. From your description, it does not seem like it's going to be floating or you'll end up with a waterfall over it. I agree our SLKs are designed to be weatherproof, but I for one would not want water dripping on my car or even on a towel I put on top of it to absorb such a thing -- water that travels like that has all kinds of calcium and crap in it that will mess up that finish you just had detailed -- and you'd have to start another thread then how to correct finish problems from water spots and we'll have to try to talk you through how to use a Porter Cable or some other machine to get your rocketship back in shape.
Hope all that helps some way or other. My fingers are tired. I'm gonna let them rest.