I'm a member of the local Reef Research Team. Part of our mission is to
assist in the creation of artificial reefs. Cement is your best material to use.
It will last 100's of years. Do not use rubber, like tires.
Earlier post mention seeing a local cement company. Sometimes they will
need to get rid of cement (ie. bad pour) and could donate it (maybe a write off for them). Just make sure you have molds ready for them if they call last minute. Vary the size of the balls. Remember to document the coordiants of where they are placed and monitor them through out the year(s). Taking pictures of the site before, just after and throughout the year will help in
documenting your progress. Note the species that come around and live on, in and near the balls. Involve the local dive community as well. Once they see
that something they helped build can be exciting, they will help promote more
artificials in the area. Good luck...