Bikini Trimmers can be used in the shower because most if not all of them are made to be waterproof so they are safe to use in the shower without ruining it.
I always use my battery operated bikini trimmer in the shower after getting my vag wet with some soap and water and then I use the bikini trimmer.
I've never had an issue with using the bikini trimmer in the shower but you don't want to soak the bikini trimmer in the bath tub.
The water from the shower will not harm the bikini trimmer but if you dip it in the bath water and try to trim your vagina hair then the bikini trimmer could be ruined.