Method
The tuba and euphonium both can serve as vessels for water (or for more water than is created through the act of performance). When playing through slides filled with water, a gurgling sound is created. The act of filling and/or removing water from the instrument has theatrical value, as well.
Necessary information
As is obvious with this technique, there is a high potential for water to get all over the stage. This can be hazardous to the performer and stage.
The act of filling and/or removing water from the instrument is an all-or-nothing proposition; once filled with water, this technique will occur, and will not stop occurring until the water is removed.
The tuba and euphonium both contain several feet of very complex tubing—once an excess of water is introduced into the instrument, it will be in there for a very long time! As a result, this technique is not used very often (hence the listing of only one work in the example section).
Notation
Any clear and consistent method for notating the actions involved in this this technique is acceptable. Keep in mind that this is a one-time affair—as said above, once the water is in the instrument there is no turning back.
Relative Difficulty
Professional
Works to consider (bolded titles are particularly representative examples of this technique)
Tuba Mirum – Trevor Wishart
The Composer's Guide to the Tuba 