The HI-MaNGA survey is an HI (21cm line) follow-up program for the SDSS-IV MaNGA survey using the Green Bank Telescope. I will describe the HI-MaNGA survey, its motivations, progress to date, and future plans. I will then present new results where we combine HI-MaNGA with MaNGA's IFU data to investigate how the global HI content of star-forming galaxies relates to the mean properties of their ISM derived from optical emission lines, including integrated equivalent width, metallicity, ionization parameter, and the relative strengths of low-ionization lines such as [OI]. I will discuss if and how the properties of the ISM vary between the most gas-rich galaxies to the most gas-poor galaxies. Our results hint at variations in the amount of diffuse ionized gas and/or shock heating, which may have an effect on the efficiency of molecular cloud formation.