U. longifilia isn't aquatic, but does enjoy a somewhat water-logged environment. How did you get seeds? Virtually nobody, including me, can get the plant to even flower! I would, from an educated guess, place the seeds in a warm, wet environment... and wait.

I bought them on ebay along with several other seeds.
Thanks for the advice.

Petiolaris,
I grow longifolia var forgetiana and find it to be fairly easy to induce flowering. I raise the water level to just below the surface of the mix for a few weeks in spring, drop the water level back to normal and it flowers. It doesn't seem to flower as well if I leave it to its own devices though. If your having trouble forwering other clones maybe you could try tracking this one down.

As petiolaris stated U. longifilia isn't aquatic.
You can germinate them in equal parts of peat, perlite, LFS mix.
{{gwi:559576}}

It would appear that changing its conditions, like changing the seasons, triggers the flowering.

Steve - I didn't know you flowered this species! Wow!