Professional photographers should look into business insurance or business equipment cover. A standard camera insurance policy will generally not cover commercial use so it's best to find a business specific policy.
Professional indemnity insurance protects you in the event that an unhappy customer tries to make a claim against you or your business, whereas public liability insurance will protect you in the event that a member of the public is injured or has their property damaged as a result of your photographic equipment.
If you take your expensive DSLR camera into dangerous areas for work (if you're a wildlife photographer for example), you'll probably need cover beyond what a basic business equipment insurance policy might include.