alo100:You mean, you will consider the small rebreather dangerous to use even for shallow water? Because there is no electronics to measure the internal status? Or... ?
In many ways, rebreathers are MORE DANGEROUS in shallow water.
The faster changes in volumes and PPs (partial pressures of gasses) cause more problems not less.
Just notice that there have been a number of people die on RB in swimming pools.
For more detail you might read Bozanic's "Mastering Rebreathers", also lots of material online. A good RB course will teach and SHOW you much more.
If the goal is time underwater then a hooka rig is cheaper and far less dangerous but does make bubbles.
If the goal is no bubbles then you are talking full closed circuit RB and that is always serious money and serious training along with lots of diving the rig to maintain profiency.