I still am not clear on whether you could locate a receptacle beside the pond and plug in the 8' cord.
I would probably bury 12-2G UF as far as I could go and then place a receptacle.
As for splicing the cord, I would probably try 14 AWG cord, StaCon heat shrinks, and heat shrink a tube over the whole splice. My theory is that if the splice leaks, the GFCI should trip. Test weekly. Test your smoke detector every week too just like the directions say.
If your pond is plastic, I would make some provision to provide a ground such as bending a piece of copper pipe over the side at an inconspicous location.