Submerging the glass in water is simply an easy way to keep the drill bit lubricated with water. You could also apply a constant trickle of fresh water to the drill bit, if the glass couldn't be submerged. The idea is to flush the little bits of glass away, as they break off, and to keep the drill bit from getting too hot.

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This right here is the answer. The Rehbinder Effect basically describes the observation that surfacants (a chemical that reduces the surface tension between substances such as soap) reduces the hardness (a measure of localized plastic deformation) and the ductility (how easily a substance can be drawn) of a material, thus making it easier to deform and manipulate. By submerging the glass inside water, you can reduce the cutting force of the drill and better control the rate of abrasion. In this scenario, water is being used as a lubricant.

This is how I carve stone. I either submerge it in water or have a constant stream of water running over it to prevent the bit from getting hot.

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What pressure? You’re going to be essentially at atmospheric

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These are all wrong. The purpose of the water is to keep the bit and glass from heating up. When you heat glass locally, its expands and causes tension across the cooler areas. This tension causes it to crack. Drills/saw blades are generally diamond. You're not going to lubricate diamond. Its all about keep all the glass the same temperature.

I was a glassblower for years. I've sawn/cut/drilled many a glass.

Uneven pressure? Can you elaborate?

Also, being under 4” of water is about 1% above atmospheric pressure, which the atmosphere itself fluctuates by

Interesting. I did not realize that the coefficient of friction for diamond is actually lower than oil. That's wild.

I would also assume that the water minimises the vibrations, as a lot is absorbed?

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I can only comment from experience cutting glass, rocks, and minerals. When you’re cutting stone or glass, you aren’t really cutting them, you’re grinding a plane through the item. The cutting wheels are made by embedding grains of a much harder material, usually silicon carbide, alumina, or diamond, in a composite matrix. When you try to cut dry, the blade heats, the rock heat, and the blade starts to lose cutting grains, destroying the blade. You also aren’t removing the cut material, so it just packs down along the cut. You can cut dry, but you have to go very slowly and resign yourself to buying new blades.

One thing not mentioned is that water is often used as well to keep the number of airborne particulates down. Idk specifically about drilling into glass but I used to sell commercial grade chop saws that use diamond wheels. These would have been used mostly for cutting sections of asphalt, concrete, clay/ceramic tiles, etc. They also use a vacuum type attachment on some that try to suck up the cloud of toxic dust they create

Yes. Heat diffusion and dispersion. I cut holes in glass all the time and we only need to spray water on it to cool it somewhat.

Its a grinding application, not a cutting application. You ever oil up your angle grinder?

I believe the Rehbinder effect applies to ductile materials only.

I would go with cooling and removal of particles.

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Its for keeping everything nice and cool. Warm glass and a warm drill will cause unwanted results. glass cracking and whatever. I worked in a fish store that made custom glass aquariums and customized peoples tanks, been drilling and cutting quite a lot of glass. You use a drill and go pretty slow like ,800rpms and constantly trickle water on the drill bit and area, going nice and slow and you get a good result.

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This is absolutely the reason, thermal shock happens when glass is heated and expands unevenly and can be prevented with a flow of coolant across the area and the tool generating the heat.

Not just the bit: the material itself. Glass in particular has cracking issues when there is a high heat gradient on the material being drilled/cut

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Water is amazing at displacing energy. If I understand correctly. It's basically a giant shock absorber. And energy goes the path of least resistance which is water in this case as it displaces the most energy. If that makes sense.

kinetic energy