Sorry for the delayed reply here.
I think that 55 or even 60 degrees F water still provides a significant benefit, particularly in the shower. Keep in mind that the stimulus to thermogenesis and the cold-activated nocioreceptors is not temperature per se, but the rate of heat loss. If you lay quietly in a cold 50F bath without stirring the water, you'll certainly feel the intensity of the cold initially, but you'll soon adjust as the boundary layer of water around your skin warms up. If you occasionally move about, you'll feel a renewed coldness, especially among "protected" parts like your armpits.
If you swim in an ocean or lake even at 60F, you'll lose heat much more rapidly than laying in still 50F water, because of the convection -- the movement of water -- that strips the heat from your body faster. And if you take a cold shower with a strong flow of water, you'll similarly lose heat faster than laying in still water.
So you if you can't get the water as cold as you'd like, just make sure the water flow rate is turned up high, and really immerse yourself. For maximum benefit, keep moving around to expose all your sensitive parts, such as your head, neck and shoulders, and your hands. The more you do of this, the better you'll adapt.
Hope that helps.