I wouldn't think a regular tennis ball would be a good idea due to the materials used in its construction. It may be possible to use any hard plastic type dog toys but again it depends on the materials used to make the product.
Check the specific toy info and see if it is labeled non-toxic AND will also not dissolve or deteriorate when wet.
You may also need to look at the construction of the toy if it has any holes or openings that could possibly trap fish inside. There are a variety of toys that may work for you but I don't know of anyone that has used any specific toys.