It is not unusual for a person with dementia to dislike showers. There are a number of reasons this might be. The aspects of a shower or bath (being unclothed, feeling cold, a fear of falling) might be more stressful to them than before. With changing senses (such as vision), water might just be too confusing–you can not see it, but you are feeling it hit your head and skin. The experience might be overwhelming.
Help your spouse feel comfortable, by keeping the room very warm, using safety features like hand rails, possibly using a hand held shower attachment. You might even try keeping on your loved one’s underwear. Consider what the former routine was. Did your loved one usually take a shower in the morning or at night? Before breakfast or after? Create a regular routine that includes some pleasant aspects, like special soap, favorite music, and ice cream afterwards. If all else fails, find creative ways to get clean such as washing up in the bathroom, getting hair washed at the hairdressers, or going for a swim.