One thing to keep in mind (or a word of warning) with that cart design.
When transporting to the water, with the boat moving forward, if you come to something that will stop the wheels from rolling (rock, stick, root, roadkill possum...), the cart will stop, boat will want to keep going forward, those long levers thru the handles will apply all the torque right to those handles. To me, just the act of rolling looks like it would also want to torque on the handles.
A simple fix would be to put a eye bolt out (or just drill a large enough hole) towards the end of the shelf the boat sits on both sides of the boat. Then run a tie down strap thru one eye bolt bring, both ends of the strap over the boat, run thru the eye bolt on the other side and tighten. Run the strap forward of the raised blue trim part on your boat, that will keep the strap from sliding to the back and loosening. That way when the strap is snugged up, it will keep the cart tight to the boat and stop the axle from wanting to rotate around the handles.
You could just loop the strap around the vertical 2X4, as long as it goes under the shelf (so that it cannot slide upwards) and get the same effect
Good looking boat.
DM