The manufacturers of these products are likely to tell you that in order to work, they need to be held up and clear of any water, including splashes and waves. FWIW, the dive container itself costs in the range of $300.00, bringing the cost of a Fastfind w GPs and a container to about $900.00. The ACR is less costly and if it fits in the McMurdo container you can get them for about $600.00.
I think having a PLB when diving is a great idea for those with the cash and willingness to deal with carrying them but I also think for most recreational divers, a signal mirror, dive alert and surface marker are sufficient. The real value of the PLB would be to alert people that you have gotten lost when there would otherwise be no one looking for you and no one around to assist. Most of us are not often in that situation and really just need something that makes us easier to find by by those already looking, or present in the area.
I like to think more practical epirb solutions for divers will be available shortly, but I have thought that for a few years now.