This is assuming you're not trying to hear the sound clearly as dialogue (eg. a water nymph or something). I just did some playing around and this sounds pretty good to my ear:
Low Pass: Frequency 100-300 (depending on how muddy you want it; the lower the number the muddier), Q 0.71
Reverberation: Effect Mix 50, Effect Level 8dB, Brightness -50, Type - Tunnel
Could also add a very small pinch of AUDelay.