Depending on how heavy the infestation is, water may not be enough, but orthene will be enough. I was always shy of the stuff, but had to use it this year on a nasty outbreak of the little suckers. Not even a loss of dew or a bent tentacle on my most sensitive Drosera or Utricularia. I now recommend the product without reservations.
I have grown Drosera underwater for a month, no problem. The issue is if you have aphids on one plant they are likely hiding in others, ready to reproduce and reestablish. Orthene is a systemic and the plants become poision to the aphids over time.