How taste is percieved wouldn't necessarily change, but types of foods or tastes that haven't been encountered before or that aren't normally eaten could, possibly, taste like nothing
This is an intriguing question, but before I go into it, I'm going to assume Homo Aquarus may have the same the same diet as dolphins, but that they actually chew their food (if they didn't and just swallowed their food whole, then you could end up with them potentially not being able to discern any tastes besides saltiness, just like dolphins can't)
As far as the diet of dolphins go, they are carnivores, meaning that they pretty much eat the fish and ignore the seaweed. If the mermaids have the same diet, then it's possible that they might not be able to perceive "sweet".
Omnivores from beagles to grizzlies can detect a wide range of flavors and enjoy the taste of sugar. But other mammals with narrow carnivorous diets have been subjected to evolution’s “use it or lose it” decree. These meat-eaters are genetic mutants without working taste receptors for sweets.
One thing to keep in mind that a major reason for taste is to determine what's good/bad for the animal to eat. For something that an animal doesn't need to survive or something that the animal gets second-hand from their carnivorous diet, then it's entirely possible that things like apples would just taste like nothing to them (and therefore be unappealing to eat).
One Other Thought
Another thing that came to mind when I was considering this and considering how some animals can't taste certain things is synesthesia (a condition where a person's senses are "crossed" so that, for example, they don't perceive sweet as sweet, but maybe as the color pink). I'm not sure if it makes sens to incorporate this into your scenario, but perhaps if mermaids can't actually taste the food when they put it in their mouths, then they could see it.