Hi,
Use Menu -> Copy/Send/Share/Print -> Send/Share/Print message text when you have a message open. Then choose your printing app from the app selection window (i.e. where to “send” message text).
A variety of printing apps is supported, but I especially recommend Cloud Print by Paolo Fernandes, which can print formatted messages, including images.
Since version 1.4 AquaMail has support for Google Cloud Print, which is built into Android since version 4.4. The relevant menu item in the same place, under Copy/Send/Share/Print.