Typically there is less growth in fresh water than in salt water. For that reason, bottom paint used in fresh water usually have less active ingredients (copper, zinc, etc.) to prevent growth. Most bottom paint for salt water can be used in fresh water. However, there are some exception, such as, Interlux Micron 66. This paint is limited to salt water only. Using Micron 66 bottom paint in fresh water, or where there is a lot of fresh water run off, will result in failure of the coating.
Consider using a bottom paint that is good in both fresh water and salt water. All of Sea Hawk Paints bottom paints are good for either types of waters and are considered both fresh water bottom paint and salt water bottom paint. You never know where you might travel that will have significant fresh water run off!
Keep in mind that you probably don’t need the top performing paint like Sea Hawk Biocop TF bottom paint if you are in fresh water. You can go with a less expensive alternative like AF-33 or Silver Bullet. Anything more is probably over kill!