Sounds like the JSP didn't expand.
This link, provided earlier by JTQ (thanks again for the link
)...
http://www.gsgroup.co.za/articlepvdw.html
...presents an excellent easy to understand description of why bullets with meplats produce greater penetration than their round-nose bretheren. It is a definte "must read" IMHO.
Supercavitation eliminates all viscous drag components (save for a very small amount at the stagnation point) leaving only inertial drag components (these are governed by the material's strength properties) to slow and eventually stop the flat nose bullet.
If the .357 125 gr. JSP @ ~1600+ fps failed to expand, its flat frontal surface will act like a meplat and produce cavitation (but only at velocities in excess of 500 fps).
In such a case, MacPherson's penetration model indicates that it would've been capable of producing at least 38.5 inches of calibrated ordnance gelatin penetration which means that (using the penetration conversion factor ~2.5x for converting gelatin to water penetration) you would need at least 95-100 inches (about eight and a half feet) of water column to bring it to a halt.
Maybe it is time to start shooting into stock tanks...