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Count the seconds. In most jacuzzi scenes, the part actually taking place under water is short — a stretch of foreplay, some shots of shoulders and steam — and then the performers move to the rim, the deck, the steps or a towel on the ground.
That is not a lack of ambition. Water is one of the hardest things a crew can shoot in, and almost every convention of the genre is a workaround for a specific technical problem.
Nothing under the surface is where it appears to be
Water bends light. A submerged body sits at a different apparent position and size than it occupies, edges shift as the surface moves, and any wave breaks the image into moving distortion. A camera outside the water looking in cannot hold focus on something moving under a disturbed surface, because the surface itself is a lens that is changing shape several times a second.
The surface is also a mirror. From a low angle it reflects the sky, the lights, the crew and the room, and reflections are far brighter than anything beneath them. So a shot pointing into water is fighting a bright moving layer sitting on top of the thing it is trying to show. Polarising filters help and cost light. Shooting steeply downward helps and gives you an angle nobody wants to watch a scene from.
Put those together and there is a simple reason so much of the coverage sits above the waterline: above the waterline is where an image can be controlled.
Steam is worse than water
Everyone anticipates splashes. What ruins the day is condensation.
A cold lens over hot water fogs on contact and keeps fogging. A housing that solves the splash problem creates a sealed volume of air that fogs on the inside where nobody can reach it. Cameras and lights left near a hot tub for an afternoon accumulate moisture in places that dry slowly, and chlorinated or brominated vapour is corrosive to contacts and coatings.
The practical answer on most sets is to keep the expensive equipment out of the environment, which means longer lenses further back, fewer angles, and shots designed so the camera never has to get close to the surface. That flat, slightly distant look common to these scenes is a consequence of the equipment being protected.
Sound and skin
Moving water is loud, broadband and continuous, and it sits exactly where voices sit. Jets are worse. Dialogue recorded beside a running tub is largely unusable, which is why scenes in this setting tend to be quiet, wide and cut to music, or turn the jets off between takes and rely on room tone that does not match.
Then there is what heated water does to people over a shooting day. Skin wrinkles within minutes and does not un-wrinkle quickly, so continuity between a dry shot and a wet one gets tight. Makeup does not survive immersion. Heat is genuinely tiring under lights, and a performer who has been in hot water for an hour is not going to give a strong take, so time in the water gets rationed deliberately.
What this tells you about a scene you are watching
Sustained, well-lit action fully under water almost always means either a purpose-built tank with lighting rigged into it, or a very short shoot with a housing and a small crew. Both are unusual and both look different from the standard version.
The standard version is a hot tub used as a set dressing: the water establishes the location and the mood, the performers get in, and the scene relocates as soon as it needs to hold a shot for any length of time. Once you notice the relocation you notice it everywhere, including in mainstream film, which handles the same problem the same way.
About the original list
It was a set of free-platform links with a paragraph each, and none of them could be located now. Nothing was rebuilt from it, because a list of that kind describes copies rather than releases and copies do not last.
If your interest is in doing this rather than watching it, the water raises real practical questions — lubrication, barrier methods and infection risk — and those are dealt with in the article on sex in pools and hot tubs rather than repeated here. For the scenes themselves, producer catalogues are browsable through the porn channels directory, and the adult VOD section indexes by release with credits attached.