Alcohol decreases testosterone massively, but after sleeping it off, your regular test levels feel like superman levels. This is why hangover sex can be awesome, and any girl who left the night before because you were shooting pool with a rope missed out on the glass cutter you had the next morning.
Alcohol dehydrates you. It's a powerful diuretic. This is why you piss and piss while you drink, and the next morning why your head pounds and your mouth is like a litter box. Alcohol in no way makes you retain water, and strength gains related to water retention are dubious and anecdotal at best.
Strength gains in the gym after a night of drinking can probably be attributed to the fact that you're still drunk. Alcohol does strange things to the central nervous system, and after sleeping one off you still have some CNS depression, and it allows you to push harder than normal. I've seen plenty of booze fueled feats of strength, and reduced judgement and CNS feedback have always struck me as the most likely cause.