Referrals are always a great way to go. If you know other boaters in your marina, ask who they use, but also ask about what the divers do for them. Ask about how they charge and what is included. Many divers will include replacing zincs in their dive charge, but the zincs are usually extra, so see how much the zincs cost. 

Make sure the diver cleans and inspects the bottom and replaces the zincs, leaving the old zincs behind for your inspection. This will help you figure out how often the zincs should be changed. A good diver will not change zincs that are still in OK shape. 

The period between dives depends on your marina and the amount of stray electricity in the water. I usually hire a diver once a year and have the boat hauled once a year. Generally, zincs need to be changed at least every six months, less in fresh water.

Some divers take photos of your boat’s bottom. I had a diver inspect and clean a paddle wheel on my Raytheon depth sounder (which corrected our speed through the water). He showed me before and after photos on his camera. He apparently always does this for his regular customers and for visitors, too. This is a great service.