the concrete should not erode (at least not for any noticeable amount).
*my understanding*:
the only reason concrete is sealed by many users is to prevent the water pH changes which occur.
From my understanding, the best method is a SPRAY of vinegar (to "open the pores" so to speak) and then a WATER bath to accelerate the curing as much as possible. Again, as kgb said, this only happens UNTIL THE CEMENT IS 100% CURED. This DOES take some time, which is why people do baths/etc to speed it up.
On the other hand, you can just use an epoxy and "seal" it.....to emphasize: the sealing is not meant to keep the water/concrete separate; rather it is meant to keep the not-yet-fully-cured concrete from touching the water. There is a subtle but distinct difference. If the uncured concrete never touches your water, you don't have any water pH issues....and you don't have to wait weeks for the curing to "complete".
ONCE FULLY CURED, CONCRETE/CEMENT is PERFECTLY SAFE UNSEALED.
This is my understanding of it all...please correct me if I'm wrong as I don't have an in depth grasp of the science yet
EDIT: @HazedSHaman: Are you making the coats thicker each time? I haven't tried yet (just got my foam carving knife today...finally...) but from what I've read only 3-4 coats of increasing thickness are needed. 7 sounds like a lot!