Typically insurance is an after-market cost for cargo, but would normally be included in the mortgage costs if you don't own the ship. That would be the same as a person with a car has to carry collision insurance (in the US) so long as a bank owns the title.
Freight is insured separately, and it would be (possibly) in two parts. If you are shipping freight owned by someone else they could buy a policy on the value of the freight for themselves - if you were to lose it they would get paid but you would not get paid your profit (potentially) for transport. If you were to buy the freight on spec then unless you purchased insurance post-sale, then you'd have no insurance on the freight's value if something were to happen to it. Your ship policy would cover damages to the ship, but not the value of the freight itself.
Of course, it's entirely possible that some people would take on extra mortgage costs to cover freight, but that gets a bit complicated and costly. The reason for that is insurance is based on value of what is being insured. A load of steel is less valuable than a load of electronics. So your insurance company would want a higher premium for a higher value cargo. A mortgage payment is flat and set, thus unless your cargo insured value was fixed, it could not pay for more expensive cargos in a way that an insurance company would feel comfortable with - they don't like risk unless they are being compensated for it.
However, if you want to make it very easy, then sure, just assume all ships carry cargo insurance and the premium is part of the mortgage.