The general concensus is this:
Ecchi: Sexual innuendo and/or nudity without portrayal of sexual acts such as masturbation, penetration, etc (groping falls under this category depending on the context)
Hentai: The portrayal of actual acts of sex including penetration, masturbation, and etc. Depending on the context, some things may be placed into this category that normally would not.
A girl sitting in a bathtub nude is ecchi.
A girl sitting in a bathtub nude, and is playing with herself is hentai.
The scenes depicted in the manga that run the line consist a bath scene (nudity, but no sexual acts - ecchi), some scenes involving dream-land S&M (not very graphic, fine line but ecchi), and a scene with Sasha suckling a breast in one frame (fine line again, but due to the context and rather mild nature of the depiction - ecchi).
Most people would say that its basically Ecchi until someone whips out their d!ck and tries to put it somewhere other than their pants, or when a girl starts putting things in there.