Seems like your making an awful lot of trouble for yourself over a simple requirement. Stick it in something waterproof and tie it in the boat somewhere. Or, put it in a small Ziplok bag and duct tape it somewhere on the inside of the boat. I had mine in a small waterproof plastic thingy about the size of a cigarette case with a lanyard on it inside my little orange boat box which was always with me in the boat. Had our fishing licenses in there as well.
I have a small drybox left over from my kayak days, my phone and wallet go in there when I am on my boat. The phone thing is becomming accepted everywhere. Friends with the local sherriff, and he has confirmed that most everyone is accepting pics of documents for most things as long as all pertinent information is shown. Worst case might ve having to show up at a later date with the actual document. Has worked for me with auto ins papers, my atv reg while on the trail...my boat reg. and even my gun permit !! Just be sure you have the actual documents availiable in case your challenged.
In Tennessee we can now order plastic reg cards the size of a credit card. They are firm and waterproof. I keep one in tackle box & ordered a second to keep in billfold. Put billfold and phone and cash in waterproofi box when launching.
