im pretty sure crypts wont flower underwater.
have you any proof that they do?
Most aquatic plants are only submersed naturally for part of the year and so time their flowering to the times when they are emersed (it's a bit hard to get pollinated by insects when your flower's underwater!!!).
The groups then that you really need to look at IMO are those that are constantly submersed like vallis or that produce floating flowers like Aponogeton and Nymphaea (though the lilies usually grown in tanks are a bit hard to flower AFAIK).
The only other ones are Anubias that do occasionally produce flowers underwater.
You just need to keep all of these in really good health to get them to flower AFAIK. Also increasing the amount of potassium may help, it does in many terrestrial plants.
Aponegetons are easy to get flowering, also echinodorous species. (above the water again)
For low level, you could try blyxa, anubias & saggitaria.
Lots of ferts & CO2 help as always, but it is possible without these. Just make sure everything is non-limiting