Otherwise, it will be 'filling' to that level (or higher, if you don't have a proper vacuum breaker).
In our boat, as with many boats, the "lip" of the toilet bowl of the head is near the waterline. Some boats' toilets (unfortunately) are deeper, and some aren't quite that deep.
Water (even filled with some shit) seeks it's own level, as us White Trask all know :^))).