I planted mine just above the sand a few feet back...cutted 2 trees down and I shoveled it out, then buried the coconut...that is 100% of growing...I also planted one where you said ^^ and it failed..hmm
i've always had luck planting them on the grass right where it meets the sand. so they're not actually being buried *in* the sand. all 3 of them sprouted. hope that helps!