An Unterwassergeysir is probably the same construction - just underwater (>Unterwasser), which doesn't "boil over" the surface of the water.

Perhaps the Unterwassergeysir is discontinuous

Yes, I think you could be right

Bodensprudler are underwater jets for the soles of the feet and ankles. If you look at the picture of the outdoor pool from the link I posted before, you'll see the Brodelberg in the middle of the pool, which kind of does look like a small volcano.

Thanks Renate

Here's a picture, it's kind of like a whirlpool in the middle inside a main swimming pool, where the water bubbles up - http://www.sauna-show.de/A.htm (see Aussenbecken)

I posted this as an addendum to my answer, but just in case you don't see it! http://www.arriba-erlebnisbad.de/baeder/erlebnisbad/index.php?navid=3

I don't think its a whirlpool

Sounds like a kind of giant whirlpool.