Checked my S7 -- yep, settings -> apps -> default has no category for Email.
But it all depends on what you mean by "the default app".
Two things come to my mind:
1 - Use AquaMail when you tap an email link somewhere (e.g. a web page in a web browser):
Android will prompt for which email app you'd like to use, and there is a way (buttons) there to set your choice as the default for later, and then it won't prompt anymore and go directly to AquaMail.
2 - If you use AquaMail with a Gmail account, stopping the Gmail app from checking mail / notifying:
System settings app -> accounts -> Google -> tap the @gmail account
You will see the list of various Google things which your phone "syncs" under that account, like "Google Fit", "Google TV", "Google Newpapers", .... "Email", with a checkbox for each.
Just turn off "Email" there and you're done.