A probe that does not sink

[Streetwind]

For a simple flyby mission of a few nearby interplanetary destinations, I built an unmanned craft. It is supposed to return the probe core and attached science to Kerbin. Currently the re-entry stage has an Okto-2, a Z-200 battery, a small reaction wheel, a Mk16 parachute, and a size 0 heat shield. It is aerodynamically stable, which is good because I play with "require signal for control" and "plasma blackout" difficulty settings active.

The problem is, when it splashes down in the ocean, it sinks. This is a problem because I also have the "part pressure limits" difficulty setting active, so the sinking probe gets crushed after less than a hundred meters of depth. I cannot recover the probe unless it either floats or rests on solid ground/ocean floor. Decoupling the heatshield does not help, the probe still sinks just the same.

Since this will be coming back from interplanetary space with little to no remaining propulsion, I cannot control where it comes down. And I don't want to just roll the dice on a multi-year-long mission, only to lose it all after successfully returning to Kerbin.

How would you build a re-entry vehicle for a probe that a.) is aerodynamically stable, and b.) floats on water, and c.) returns science observations intact?

[VoidSquid]

Yeah, I had that issue too, my two workarounds: either a bit more fuel for small last minute course corrections, or alternatively I added a MK-1 pod to keep the thingy floating, I prefer the 1st method.

To guesstimate when the probe might land, the Trajectories mod is pretty helpful.

Edited October 25, 2019 by VoidSquid

[Xd the great]

I add an empty fuel tank/mk1 structure tube. Adding rear fins may help stability.

[Streetwind]

After deliberating, redesigning, and test flying quite a bit, I went with @Xd the great's solution: an empty mk0 liquid fuel tank. Managed to integrate it without losing passive stability, and the probe now floats in water. The added mass is quite small. Though admittedly, my probe looks a bit phallic now

[Klapaucius]

Wow, I've never seen this issue before. As someone who has built a few submarines in KSP, the challenge is getting things not to float.