When are Kerbals 'in proximity' to sunken craft, when not, and what determines, leaving it, the speed of ascent?

[Hotel26]

No doubt, the rules on this have changed over game versions, but I'm particularly interested in 1.12 (which is near enough to 1.11, which I play).

This mechanism is important when trying to swim from one craft to another one, very close by, in order to board it.

Kerbals seem to have a loiter-in-a-very-slow-climb mode that is quite annoying if they escape and begin crawling to the surface.  Call that 'crawl' mode.

Then they have a 'shoot' mode in which they rise rather rapidly.

Spoiler

To rescue them early, it is quite a good game to maneuver a rescue craft into their path overhead (either mode/speed) with a deck chair to 'save them from certain death' (from e.g. air supply exhaustion?  ).

I have built underwater base experiments with e.g. 'verandas' or overhead arches to provide a 'safe zone', but don't really know what I am doing without knowledge of the 'rules'...

Ideas?

Edited September 2, 2023 by Hotel26