Ocean swimming doesn't contribute to weight loss?

My Sim is a mermaid that lives in Sulani and she swims A LOT and has a decent fitness level because of it. She also eats well, including when extra food is brought over, but her weight and figure appear to be the same as when I made her, which I'm okay with. I'd be less okay if I had a Sim I was trying to get into better shape and nothing was happening despite all the swimming.