Most insurance companies will allow you to have an "agreed value" for your trailer, meaning that you can insure it for what you want within reason. I would make sure you insure it for enough so that you can replace it if it were to be stolen. I don't know what kind of trailer you have so can't tell you what its worth.
Same with your boat, don't let an insurance company insure it for less that what it would cost you to replace it.
Don't know f that helps much but I tried!