That bubble ability works 10mins/day.
All the talk about fishing sounds pretty cool as a survival tool
I can just spend 10 mins inside a small river with a bucket waiting for that "fat, juicy wriggling fish"!
Anyway, yes, the character can't rely on water to float, so a downside is that if he's dropped into a lake while the bubble is active, he will fall straight down to the bottom, possibly taking fall damage. It doesn't mention any kind of drag or resistance when in contact with water. It seems that water simply veers off, so a decanter flushing out liquid with the geiser ability against this, would probably have no effect.
So, to cross a river, the character would have to climb down to the bottom of the river bed from the bank on his side, walk along the bottom and climb up the other side.
On mud, i think it would either push it away, if it was too liquid, revealing the bottom underneath, or dry it off, providing a somewhat solid surface to step on.