The classic commentaries such as Rashi (Genesis 1:7) Nachamandies and Ibin Ezra (ibid 1:9-10) mention that the first "it is good" is connected to the day before. On Monday G-d did not say "it is good" even once. The reason they give is because He hadn't finished establishing the parameters for the water; only vertical parameters were set on Monday. On Tuesday, after establishing the horizontal parameters for water, leaving space for dry land, the creation of water was complete and G-d said it was good. Then He went on to create the vegetative life on dry land, for which He stated it was good as well. A more homiletic interpretation is found often in Chassidus based on the Talmud Tractate Kidushin 40a "Good to heaven, and good to creation" - the double term of good on Tuesday is meant to imply that it is good spiritually and good physically; good for G-d and good for people. (See Likutei Sichos volume 20 page 368 and footnotes there).