a frog leaps in--
There's something that is both specific and universal in the original, and I'm not sure that the indefinite article does that ambiguity justice. Natch, I'll have to try it out with the definite article one of these days (though I'm not so sure I'll like it).
What I do like is that last line--"water's sound" is, to me, the closest English cousin to "mizu no oto". I've always loved the Japanese no--the possessive article--because it replaces "of" in the position of the possessive "apostrophe-s". In Basho's poem, the assonance of the "o" sounds in the last two words is particularly lovely and particularly indicative of the sound of a frog slipping into the surface of a pond. The long "i" and "u" in mizu supplies the surprise of the movement. Granted, it's still a quiet surprise, and that's why I find the short "a" and schwa of "er," the elided esses and the rounded end of "sound" to be a happy relative to Basho. The vowels are all quiet, but they all vary. They are the quiet surprise in a newly troubled lake.
More another day about the ya...