Again, onomatopoeia, but more than one kind this time. From what I can hear it seems to be: jaba jaba basshaaa zabun zapua!
jaba = splash (see also shapu, zabu, and bashan, picha, pisha for smaller splashes)
basshaaaa (バッシャアアアア) = Big splash, like from a big bucket of water being poured. [Contributor: Humpty; Ref: ♭37℃ by Tsukino Jyougi]
zabun = big splash (see also jabon, shapu, and bashan, picha, pisha for smaller splashes)
zapua (ざぷあ) = splash. [Contributor: Humpty. Ref: ♭37℃ by Tsukino Jyougi].
Or something like that. We'll find out when the toy scans come out.
Reference:
http://www.tonigobe.net/