How can I get a list of all the characters containing the "flowing water" component that is in 益?
The top part of 「益」 is just a 「氺」 (水) rotated 90 degrees.
秦簡12.47睡虎地秦簡篆皿部說文解字東漢隸華山廟碑　楷　
秦簡25.46睡虎地秦簡篆水部說文解字東漢隸郙閣頌　楷　
This makes the top part of 「益」 just a variant of 「水」. Predictably, the character 「㳑」 is the variant with more standard components, although this character itself is not standard.

Normally, a search for a list like the one in the question would also incorporate everything containing 「水」, just like a search for everything containing 「水」 would normally include everything containing 「氵」.
If you only want characters containing an arbitrary component 「Ｘ」, in general, I would advise against going through an ideographic description sequence decomposition database like CHISE or Pleco's component search containing 「Ｘ」, at least not without some due diligence checking later. The results which turn up are very misleading if you're not careful. For example, the search for 「⿱䒑八」 in CHISE literally turns up everything which

contains regular script strokes which look like 「䒑」, an abbreviation for many things including 「艹」, on top of
a bunch of regular script strokes which look like 「八」

「水」, written sideways or not, does not contain 「䒑」 and/or 「八」. There are other  characters which contain the stroke sequence 「⿱䒑八」 like 「」, which is an abbreviation/corruption of 「檐」.

A cursory glance at some of the CHISE results shows that the number of encoded characters containing sideways 「水」 is quite limited. Apart from everything containing 「益」, that database lists

「」 (variant of 「滅」 where the 「水」 is shifted to the bottom and rotated sideways)


A few characters containing 「」 (variant of 「涉」 where the 「水」 is shifted below 「止」 and rotated sideways)


「」 (variant of 「瀕」)


「」