I'm a little late to the discussion, but I just wrapped up a trip to the beach and decided to use said travel plans as an excuse to purchase a pair of SP29K-M.
Comparing to SP29-M, there are a few quality of life upgrades. Similar to P34-E, there are small patches of GoreTex that serve as reinforcement in high-stress areas. And similarly to P24A-E, there are loops on the interior of the shorts that you can hang them up with. In addition to the mesh pocket bags, there are also small drain holes on the lower cargo pockets that are far less effective at emptying liquid compared to the interior pockets, but still help make the shorts more suitable for swimming. Lastly, the (much lighter) fabric has a ripstop weave. The necessity of this is debatable, but a slight surprise as it doesn't show in photos.
The negatives first: The excess length on the integrated belt is annoying in water. The retainer loop does not help much and while you can cram it into a pocket, it won't stay there for long. Maybe a minor nuisance that will affect some more than others, but this is my biggest complaint. Secondly, the pocket array. It's just overkill. I did find some use for it by being able to swim in the ocean with keys to a car (with no worry about loss) but no pair of swim trunks need all this storage. And when you're not swimming, storing any item larger or heavier than maybe wireless earbuds can be pretty uncomfortable for walking around or sitting.
The positives: The fabric used on the "K" variant is much lighter than it is on SP29-M which makes for a much better pair of swim trunks but more importantly a better pair of shorts as well. Whether or not you like the metallic look or not will be up to you, but I'm indifferent and it's not too shiny in person. The shorter length is also much more summer appropriate and in my opinion, comfortable. Of course, they also look cool.
So if you're searching for something to swim in, you can probably look elsewhere. But if you want a pair of summer shorts that can be used for swimming in a pinch, these are great. Especially when you can get them on sale from HBX for "only" $330 USD. Overall I'm pretty happy with my purchase and wish I had just skipped SP29s in SS22 and waited for these