The two most popular insurers of underwater photographic equipment are DEPP and H2O
Both offer insurance that covers the flooding of your equipment. However, only DEPP insurance covers the loss of your rig if it is lost while diving e.g. the DM accidentally looses his grip on it while handing it to you on a wall dive.
Other differences in the policies are that DEPP replaces the equipment for you with equivalent equipment whereas H20 pays you cash for the replacement of your equipment. And DEPP requires you to insure a basic scuba rig to qualify for insuring your camera whereas H2O has no such requirement.
As to using ones homeowners policy, views are mixed. However, the biggest concerns seem to be uncertainty as to whether flooding damage will be covered and whether a camera claim may trigger an increase in ones total homeowners policy.
Bottom Line: The reason for getting insurance is to protect you against losses you cannot or do not want to pay for out of your own pocket. So how much insurance you need depends on how deep your pockets are.