For drowned, you know that that while they are capable of swimming and climbing, they aren't otherwise too different from your typical zombie. They’re tough to kill by conventional means but holy magic can put them down very easily, though again this is pretty obvious for anyone who has ever thought about dealing with the undead before.
Sirens are a bit more of a mixed bag. Firstly they are a Fey creature and while it’s not unusual for evil fey to command the undead, sirens as a whole aren’t typically “evil”. Granted they are fey, and fey have a very alien outlook on good and evil compared to mortals. Usually whenever you’ve heard stories about mortals succumbing to the song of a siren, it’s usually due to the mortals own carelessness and pride rather than anything the siren does beyond her alluring song. More importantly, sirens are vain fey, and are attracted to uniqueness and beauty. It’s not unusual for a Siren to this claim a shipwreck and it’s treasures, adorning themselves with things like jewelry and the like, but at the same time they are very disgusted by things they consider ugly, which is usually people and the undead. So a siren at the shipwreck? Normal. One control undead? Abnormal.