depth of water does not affect humidity. The surface area of the water is what affect the humidity.
In simple terms the larger the bowl the higher the humidity the smaller the bowl the lower the humidity.
High humidity will always travel to lower humidity so if your home has low humidity then the humidity from the incubator will escape to the home. removing vent plugs can assist with dropping the humidity. So ventilation is good for the exchange of oxygen but large amount of ventilation will cause the humidity to drop. (unless you live in a humid place then it will work the other way)
so what happens when most of the vents are plugged and you can not fit a larger bowl. Well there's a easy fix just take a sponge (they sell special sponges but a standard one will work) place part of the sponge in the water and have the rest standing up in the bowl. A sponge has lots of surface area water will soak up into the sponge and the humidity will rise.
Low humidity is a nice problem to have as its a easy fix.
P.S being a still air the humidity tends to be lower as the fan in a forced air will assists with stripping the water molecules that are bound by surface tension away from the water. In simple terms a fan blowing across the bowl of water will make high humidity.