You are right. When you put the pasta in water, the water molecules diffuse into pasta and expands it. The boiling medium is the hypotonic solution here; as the concentration of solutes is less in it compared to the cells of the pasta. Tonicity is a relative measure between two solutions, so any real world example would require two solutions with an osmotic gradient between them. In the pasta example, water is the hypotonic solution. And for the cells found in the fish gills, the salty seawater is a hypertonic solution.
