How to make a round terrain?

I want to make an earth and I don’t know if I should do it in a modeling application. If I do than how will I add water.Should I make two meshes? One for water and one for the terrain?

Depends what you want to do with it.

Trying to make a round object in unity would be very frustrating and tedious since the terrains are flat squares.

I say 2 meshes that way you can have the water a different material from the land and scaleing the water wil give the effect of a global flood or drought.