It is always best to use a separate Inland Marine policy, not just an add on to your homeowners' insurance. If there is a claim, then it would not effect the premium if the items were insured under a separate Inland Marine policy.
Inland Marine policies are available for both personal and commercial coverage. If you are not a professional, get the personal inland marine policy separate from your homeowners. If you are a professional, then either a separate commercial inland marine policy or as a rider to your CGL [commercial general liability] policy. Commercial packages for liability and business operations go by many names specific to each company such as Flex, Spectrum, BOP [business on premises] Five Star etc. Just a name for essentially the same coverage.
Most of your major insurers offer both types and some of the companies that write this kind of coverage include The Hartford, Erie Insurance Group, Chubb, MetLife Auto and Home and State Farm.
Keep in mind that if you write the coverage as a rider to your homeowners' insurance policy, then it will increase that premium, and if you have a mortgage, then your monthly payments, principal, interest, taxes and insurance will increase. And, if there is a claim, the cost of your homeowners' coverage will also increase.