Its also worth mentioning that some of the mermaid abilities are quite useful buffs like the Siren's Lullaby which is a great way to either buff Sims (especially if you have two mermaids that can buff each other) and remove some negative modlets.
Charmer's Lullaby - Removes Embarrassed Buffs and gives a +2 Flirty Moodlet
Inspiring Berceuse - Removes Uncomfortable Buffs and gives a +2 Inspired Moodlet
Aegean's Question - Removes Focused Buffs and gives a +2 Stressed Moodlet
Night's Requiem - Removes Energized Buffs and gives a +2 Sad Moodlet
Then there is the Aquatic Lure which makes you fish at a couple of levels higher.
Even Siren's Call gives a Sims a stressed moodlet will also refill all of your Sims motives so being a naughty mermaid has its benefits too!