I went through this with camera equipment years ago. I had a personal articles policy with a major company and at that the time all they asked for was proof that you had the item in question which meant either a picture of it, a receipt or a box and you that you brought the items to the insurance agent to confirm the serial numbers.
After about 5 years from when I signed the policy, I made a claim when I fell down a small cliff and my equipment went into a lake. It was a loss of about $8000-$10000. The insurance company investigated it like it was a scam. They tried everything to deny it. I had to file a police report (which the police laughed at> no crime) and I had to go with the insurance investigator to the place where it was lost twice. After a couple of months, they agreed to settle. At first I was excited, then they tried to scam me by sending a "similar camera and lens". The equipment had a new value of about $1000. It was a joke to me. I eventually had to send them item numbers and serial numbers from new equipment at my local photo store that was either exact or similar in price. They were not going to just issue me a check despite it stating as such in my policy.
After this claim, they either wanted to immediately drop my policy or double the cost with a $1000 deductible. Also, according to them, and this is the main part related to your fishing equipment, all personal articles policies going forward must have a receipt of purchase from a reputable licensed dealer. This meant no used equipment, eBay, or dealer specials, even if new. This was in 2004.
Here was their reasoning... Maybe a year or two before I made my claim, it had become a major issue for them to have people buy used equipment, insure it, then claim it was damaged, lost or stolen and the insurance company would just replace it with new or write a check for the full retail price> especially if it was a partial loss. They just didn't question it because it cost them too much to investigate it. Eventually the claims became so frequent and high in dollars that every claim was scrutinized. What they had found out was a lot of claims involved stolen goods and people would either sell it again or pocket the money and the product with there not ever being any loss at all to begin with. People could turn an item over two or three times > This is the reason they need receipts.
