Appears to be a legitimate complaint.
If you had an agreement for full insurance, and he did not insure adequately, this is a failure on his part, and his not keeping to what is paid for: the contract.
He may be trying to claim that this is 'all the insurance that could be obtained' in resolution. However, if he did not insure the watch adequately accorind to your agreement, there is an outstanding matter.
For one, I always ask for insurance to the full value of the watch, and even go so far as that this is a condition of it being sent before payment. Oe could explicitly specify the amount to be sure of how much it is to be and should be insured for.
I hope that paypal come back positively, and that your cc company do too.
It may be worth outlining that he agreed to insure the watch fully and this was part of the binding agreement between you, therefore he doubtless holds responsibility, and ask what he says to that.
Best Wishes,
AP