So in Super Mario Bros. 3 there is a location referred to as the Grass Land or Grasslands. This place is actual the main location surrounding the Mushroom Kingdom.
In Mario Party 9 when Mario and friends head East to Toad/Adventure Road to stop Bowser, Bowser and Bowser Junior state this is in the Grasslands. We can also tell this is the Mushroom Kingdom still due to the water and hills.
What's notable is in every game when you go to rescue the Princess you head East. For example New Super Mario Bros. Wii and New Super Mario Bros. DS.
There Is A Lake Or Inland Sea To The South Of The Grasslands
So something I've noticed is to the South of the Grasslands there appears to be some kind of lake or inland sea, and it is consistent across games.
So in Paper Mario 64 you head down South from Toad Town to talk to a whale that takes you to Lavalava Island. That alongside the map shows there is a body of water here.
In Mario Party 9 to the South of Toad/Adventure Road we again can see this body of water.
In Super Mario Odyssey when lore hunting I noticed the desert to the East of the Grasslands. So I decided to look to the South and what did I find? Well of course the body of water that keeps showing up.
Finally to end it off in the New Super Games when paying attention you can see that there is a body of water to the South of the Grasslands.
Edit, due to the fact Dinosaur Land is located in this body of water this would confirm this is an inland sea, and that it's name is Cheep Cheep Sea.
Explanation