My just turned 4 year old ds does not like the shower. But on our cruise in April we did not have a tub in our cabin.
A few months prior to our trip, once or twice a week I would tell him the tub was broken and he needed to shower.
He hated it at first but I would play and look like I was having fun (I did not go in the shower) . I put the shower head on medium/light and aimed it mostly away from him while I soaped him up. When it was time to get the soap off, I would laugh and told him it was raining on him. That he seemed to like.
I would also have him hold a face cloth over his eyes while I did his hair. He liked that it "was raining on him". On the cruise, he took a shower ever day and didn't seem to mind too much although he does prefer baths. I just kept telling him it was raining in the house or in our cabin and he thought it was funny.