Alright. So. Hear me out.
There's nothing sexy about this video, but I think it represents a very good proof of concept of something that could be adapted.
1) Replace occupant with cute naked girl
2) Provide low-profile breathing apparatus linked to air supply outside so she can stay in indefinitely.
3) Modify the water to preference. Could add thickeners (xanthan gum turns water into fake semen) , colors, scents, etc. Keep in mind this may contact her eyes. Take precautions.
4) Don't fill the balloon so much, so it conforms more tightly to her body.
5) Keep the entire thing slightly above body temperature so it's warm inside. I'm open to suggestions on how; heating pads would be dangerous with this much water.
6) Seal the bubble from the outside. Give her a panic button to hit if she needs to escape in an emergency, where a partner can release her, but she shouldn't be able to escape on her own.
7) Add ancillary trimmings to taste; oblong balloon instead of spherical for snake-like or intestine shaped confinement, bondage, food-prepping her as foreplay before putting her in, etc.
8) For the ambitious, maybe fill her up with BATTERY-POWERED (important) vibrators locked under a chastity belt so she can't stop them. Could simulate a tentacle hell pit as well.
9) Backlight the entire arrangement so you can watch her squirm helplessly inside a cloying stomach.
10) Don't sue me when you fuck up and accidentally kill her.