Dog trainer/behaviour consultant here..
The risk you have is leaving him in a new location may cause distress, only you can decide if that is likely and whether its avoidable, and whether if distressed he will throw himself around or just stay still in one place.
If the shower/bathroom is the safest place you have, then it is.. as long as he has room to move around, choose warm or cool surface, has access to water and its within his threshold for being alone, I can't see how that's an issue.
For the raving lunatics blarting on about crate training..
Crating can be extremely useful for dogs who need to be pretty much immobilised for rehab, as in the case of spinal injuries.
Where we'd be able to keep a human in bed or in a brace and tell them not to move, we just can't do that with a dog so crate or a small pen depending on the dog, is the answer.
All dogs should be crate trained - by that I mean all dogs should be introduced to a crate, conditioned using positive reinforcement to feel relaxed and comfortable in a crate, and again conditioned via positive reinforcement to feel relaxed about being left short periods in a crate.
If you do not do this, then when your dog needs crate rest to rehab from injury, when they need to go to the vets and stay there during the day or overnight, when they need to travel in a crate (and you may say you won't need most of those but the chances are, your dog WILL experience at least one of those things in their life), they will not be prepared and it will cause them distress.
Like literally anything, crates can of course be misused - people often refer to 'crate training' to mean 'shut the dog in a crate and ignore it crying' - thats not how it should be done or needs to be done.