These are the stock Hitchhiker Modules, yes. I've brought them down into the water, slathered in parachutes, as slow as ~5m/s or so: very gentle landings, but they just vanish as soon as they hit the water. All the pieces connected to them survive. The tanks themselves just seem to be, as I said, water soluble, and disappear when they hit the water, even when it's a really gentle landing. I think it's an issue with this particular part. (I may try transporting the crew in the 2 man landing cans, although that won't do any good for my mass.) They have a very low impact tolerance for their size, and especially for crew-carrying parts. The ship isn't all that large. The crew return stage (which is all I'm testing at this point, I'm just boosting it up on solids to test its water survivability since I knew this would be a problem. Haven't built the launch vehicle yet.) consists of a 3-man pod, 2 hitchhiker tanks, and some basic equipment for orbital maneuvering, docking, and surviving dry landings.