I think Halperin's plan to cover up his wife's murder was actually devised after he drowned her in the tub. As she happened to be in the bath at the time it presented him with the option of making it appear that she drowned, in plain sight. I don't think he had the whole pool gag planned beforehand and she was handily bathing at the time he came home from work.

I think we assume that
(1) Halperin was confident enough that he could cover up any evidence about the soap (or he didn't realise it in advance) - the same could also apply to the time of death issue
and
(2) that by being "back early" he knew his wife would be in the bath - a regular event, perhaps?

A Friend in Deed is the quintessential Columbo (the hallmark of the entire series), and since I've waxed over every reason why in this forum at least 100 times, that's all I have to say about it.

Can someone explain to me why Halperin thinks changing Janet Caldwell's clothes somehow gives Hugh Caldwell an alibi? I mean she could have changed her clothes prior to him murdering her. Why didn't Halperin just have Hugh pretend she was watching tv when she called?
The only thing I don't like in this episode is that Columbo doesn't seem the slightest bit nervous arresting his boss, nor does he seem surprised when he figures out the Commish is guilty. I have to assume that Halperin had a sleazy reputation, since even Hugh Caldwell knew Halperin would cover up for him. That's a lot to ask of your neighbor.

We never hear Halperin ask for Columbo on the telephone after Janet Caldwell's murder. In fact, we hear him ask for a patrol car to check out the Caldwell house and then we see Halperin hang up the phone. He never asked for Columbo.
Later, Columbo asks Halperin why he specifically called him the night of Janet's murder, since the robber had never hurt anyone before. Sloppy!
I wonder if they changed the story to provide Columbo with another clue.

Agreed. I used to watch that episode over and over. It's really good! As of right now, I am watching Colombo almost every night, especially before going to bed.