Submersible DC motors are not a part that we carry or can get from any of our motor suppliers. I checked around online and submersible DC motors that are large enough to power a cart appear to not be available anywhere. The requirement of having a submersible motor might sink your plan for making this kayak cart so you may want to start thinking in a different direction.
I recommend trying to design the cart so the motor is above the water when the kayak is launched, something like this.
If you design a kayak cart where the motor and electronics are above water then we can supply the parts for a project like that.
Please let us know if you have any questions.
For the kit if you could please let us know what parts you would like included with it then we would could begin with making a part number and link for it.
Please let us know if you have any questions.
Tim H