Great. 
You can set Gravity = 0 as well, no need for that under water.
Hah, yes I see what you are expecting to have happen.
Let me try and break it down.
- First selection is your child
- Second selection is your parent
When you make the Attach Constraint, you’ll find a new child locator, parented to the parent control.
https://user-images.githubusercontent.com/2152766/180437225-b02f5973-11cf-4621-b0bb-2e8d586e4e40.mp4
You can move this around after it’s been attached, and Ragdoll will try and keep the child Marker where this child locator is.
https://user-images.githubusercontent.com/2152766/180437230-7c0621d3-98a4-4c92-96e1-0e673c1e2094.mp4
You can animate this child too, which is where things can get more complex but also more powerful.
In some cases, it may make more sense to both Attach and Parent Constrain one control to another.
https://user-images.githubusercontent.com/2152766/180437917-e12f35f0-1dd2-4457-a078-5a29fbb63a05.mp4
Notice here that now we’ve attached the box to the character in both Ragdoll and Maya.