Yup, graphs are helpful. Specially since it is a lot more work to move across water than a boat. You might be wanting to target a certain depth and a graph lets you plan for it in advance. I like this link
http://www.charts.noaa.gov/Interacti...c.shtml?rnc=50
I check it out when I want to explore a new area. You can zoom in to an area and get a pdf file that you can look at or download.