It is very possible, because scientists have only explored about 10% of the oceans on Earth. However, I do not think that they will be like the way they are portrayed in The Little Mermaid. If there are mermaids, it's likely that they live in such depths that scientists can't explore because of pressure and darkness.
Pretty much impossible. Sea creatures don't need to evolve arms or hands, since they are less hydrodynamic. Hands could have been a possibility if they could live on both land and water, but then their tails would be obsolete once on land.