I've found that this is common when you're channel surfing. When you listen for a while, the problem subsides a bit.
I'm told that this has a lot to do with the Forward Error Correction (FEC) scheme that is used by Sirius and that it doesn't kick in immediately as you can't go back in time and retrieve old FEC data. It would seem that the FEC data is actually part of the data stream.
I think they overcompress in order to add more channels, and also use a statistical multiplexer. This results in moment by moment variation in audio quality, depending upon which channel on the transponder has priority. I have noticed truly abysmal (underwater) quality on speech.
There I times I have to agree it sounds like too much compression or as the OP put it speaking under water. But it isn't all the time, it isn't on music channels, and it doesn't last long especially if you stay on the channel.