Beta Radiation Is Not Always Low Energy

Well some beta radiation is, like that emitted by tritium (18 keV), but some isn't low energy like that emitted by Cs-137 (512 keV (95%) and 1,174 keV (5%)).

Note that these are maximum energies, since beta radiation is emitted with an accompanying electron anti-neutrino.  Average energies are about 1/3 of maximum.

I don't know what the predominant isotopes are associated with the Fukushima water problem.

But I saw this sentence in this article:

"And the kind of radiation being emitted is very low energy, which means it is not able to penetrate the body easily."

It isn't the beta energy which prevents beta's from penetrating the body, though it obviously plays a role. It's the fact that beta particles have mass and an electric charge.  So they interact very readily with atoms and molecules in your body and have short track lengths as compared to photons (no mass, no charge).