The characters are concerned about the houseboat sinking, as well as the surrounding walkers, yet walkers are only waist deep and slightly higher as they are walking up to the houseboat on all sides. It couldn't sink more then it already had since Morgan is already waist deep while on the houseboat.
Morgan buries Grace near Eastman, whom he had buried well before Mo was born. Season 8 is 7 years after season 7, so it has probably been at least 10 years since Morgan buried Eastman. Yet, the WOODEN marker he placed on his grave is still erect? In the very wet environment of Georgia, particularly in that swampy area, that marker would have fallen to the ground within months. Further, it would have rotted away well within a year or two. Further, time and nature would have encroached on his grave and erased all signs of it within the following decade. So, Morgan would not have even been able to find the grave let alone find the wooden marker he had erected over it.