Some items (for example, a ball and a travel voucher) can help to pass the border checkpoint, but many other items can alert the guards and even lead the mission to failure.
And it is quite logical, just imagine, you put flippers, a boat and scuba gear in a backpack, but instead of a river bank, you went to the checkpoint... Any inappropriate things give you away. You can tell stories why you need this boat, and that you really need it, but you will already be in the isolation cell, and then in the forced labor camp until the end of your days.
Also, you will not be allowed to pass if you have a weapon with you, such as a knife or a pistol. You are most likely rounded up and sent to the owners of the boat in a backpack.
There are things less dangerous, but undesirable, for example, food. Border transition takes only a couple of hours. Of course, you can justify yourself that you always want to eat. The thing is, the only food you can get and take with you is preserves. The border guards would not believe that you wanted to eat this jar of canned meat right on the border.