That's a really great idea on how to play! For your Sulani, what about if you made an area of the water contain floating urns? It could be a floating graveyard! I think the only kind of urns that don't float are occult and pets, but I may be wrong.
My personal tradition for the tombstones/urns is to have at least one family member go to a graveyard (I have only 1 and it's across from the Goth's as a generic lot) with the tombstone/urn in order to place it there. Sometimes I will invite over extended family and friends to have an informal get together to honor the deceased.
My graveyard has both an outdoor area and an indoor basement for tombstone/urn placement, where I try to keep family groups together. In the basement I have multiple rooms of urns, a podium with seats to give a speech, a dining room with a buffet table, and even a room with vampire coffins hidden by a secret bookcase door.
I like having the basement because it is an easy way to expand storage without disturbing the lot's appearance and you can make small rooms for different families. I was thinking of making a pet section, and it will be simple to carve out a new underground room when I do.