As far as flowering underwater goes, not so much. You are limited to the occasional flower on anubias barteri and maybe even crytocoryne wendtii. Aponogetons will send shoots up to the water surface and have a flower in the air.
Amazon swords will flower but not underwater, they will send a big runner out where the flowers would be.
As stated before, most plant's don't normally flower underwater. However emersed they can flower all the time. I have found that persicaria (polygonums), swords, Limnophila's, and ludwigia's flower often emersed.
Vallisneria and Blyxa are true aquatics and must flower underwater although the flowers probably aren't much.
If you let a Nymphaea grow to the surface and develop lots of floating leaves it will flower above water. Beautiful but they don't last long.
Hydrocotyles, Rotalas, Echinodorus, Cabombas, Polygonums and Hygrophilas have flowered for me as well - above water. The only flower I have had in the water is Anubias nana, have 2 old flowers and one new bud in the tank right now.