Fat is less dense than water, so while it would hinder swimming it would also benefit floating for longer periods of time with less effort.
If we assume Mermaids are indeed air-breathing amphibians this would provide an evolutionary advantage inline with most ambush predators. Their mythology encourages that characterization well — i.e. they wait for passing ships and lure sailors into the water or encourage fishermen to ‘catch’ them before eating them. One would imagine like a snake or lizard or indeed like many predators of the ocean the goal would then be to preserve calories as much as possible. Therefore floating at or near the surface for easy breathing, heat management, and to encourage their chosen prey to spot them so they can start their ambush is better than being able to swim quickly.
Fat is less dense than water, so while it would hinder swimming it would also benefit floating for longer periods of time with less effort.
If we assume Mermaids are indeed air-breathing amphibians this would provide an evolutionary advantage inline with most ambush predators. Their mythology encourages that characterization well — i.e. they wait for passing ships and lure sailors into the water or encourage fishermen to ‘catch’ them before eating them. One would imagine like a snake or lizard or indeed like many predators of the ocean the goal would then be to preserve calories as much as possible. Therefore floating at or near the surface for easy breathing, heat management, and to encourage their chosen prey to spot them so they can start their ambush is better than being able to swim quickly.