I haven't had this particular plant flower, but I have had a similar stalk-style flower. I found that the flower itself needs to be out of the water. In one test when I left the end of the stalk in the water, the stalk kept growing longer and longer with no actual flower appearing. After the stalk was 5+ feet long, I supported the end of it out of the water and then the outer covering at the end dried up, split, and the flower emerged. Another time, as soon as the stalk was long enough to support the end out of the water, I took it out and again the flower emerged despite the stalk being very short this time. I've got a third stalk growing that I'm just about to support out of the water.
I suspect in nature the plant sends the stalk straight up until it emerges from the water and then it flowers. Perhaps the plant and/or stalk have more support in their natural environments. Or maybe there's enough other plant matter at the surface that the flower can rest on. It makes sense to me that the pollen would need to stay dry if it's to pollinate anything.