Learn English – What does “brought low” mean in this sentence

First, some definitions:

• In UK slang, wet means weak, wimpy, pathetic, spineless. This sort of thing is (or was) commonly heard in schools:

  Don't be so wet!, or

  I can't believe you won't climb the wall with us. Girls are so wet!

• A little squirt is also slang: it's a mildly insulting term for a small, insignificant person (usually a child).

So, the quote: "Already wet, aren't they?" is playing on the two meanings of "wet" - they're already wet (spineless), so it doesn't matter if we get them wet (with water). (This is more likely than the pure literal interpretation of "wet", coming immediately after the stereotypically girly "wet" behaviour of screaming and running away - Peeves's comment is in direct response to this, hence the aren't they question tag.) And similarly the little squirts also refers to the girls, in a similar act of insulting wordplay (squirt=small person; squirt=jet of water).

Tumble about means to either roll around, like sea would capture her heart and waves would roll around it, metaphorically

You are exactly right.

"tumble" here just means "to roll over and over, to and fro, or end over end", and "about" is used in the sense of "all around".

Think of waves in the ocean moving all around her heart, in all directions, crashing together (and of course figuratively!). Water is often a symbol of emotion. Here, Eveline is feeling overwhelmed and thus, at this very moment, is conflicted about whether she should go or not.

There is a word in English, "aflutter" (which means "nervously excited") that could describe Eveline's current state.

Note that a "flutter" (think of the movement of butterflies) is something we can feel in our heart:

Heart palpitations are a feeling that your heart is beating too hard or too fast, skipping a beat, or fluttering. [...] Most of the time, they're related to stress and anxiety.

I think it is this sensation that Joyce is alluding to.

(Note also that "A bell clanged upon her heart" is like the feeling of her heart jumping, or "skipping a beat", as Ronan points out).

"fell upon" is one way to interpret "tumble about", but really it's too simplistic. It doesn't capture the chaotic movement of the water, rather suggesting that it comes down in one action - which is not synonymous with "tumbling".