Question:
I wonder what this sentence (and the Spanish) mean. Does it explain a general characteristic of water ("Water is a substance that glitters in the sunlight"), or does it describe what's happening right now ("Look! (The?) water glitters [is glittering?] in the sunlight.")?

I think that the second meaning is more probable though both meanings you mentioned are possible. Google search gives some examples on poetic or descriptive usage of this sentence and its derivatives. For example, we can find the following lines in "The Book of the Thousand and One Nights":

Water glitters in the sunlight
Like tears upon a cheek in willow shade,
And flings up drops
To hang for silver bells
In the bright-hued pavilion of the flowers.

Out of context, the Spanish sentence stands for meaning #1. But in real situations, #2 is much more common. Though I'd rather say "bajo la luz del sol".

I can personally never imagine anyone wanting to tell a sentence of the meaning #1. (I guess those who are interested in the character of water would say something like "water reflects light.")

I'd like to suggest changing these sentences to the more natural ones that stand for the meaning #2, unless there are someone who insists we should keep these. What do you think?