You were right...
I checked my sea serpent this morning and he's hard as a rock. I could probably toss him across the driveway and he wouldn't even chip (okay, slight exaggeration). So I completely overreacted by panicking and setting him into a concrete base. Grrrr.
Though my wonderful husband is telling me from a design standpoint, he likes the base anyway because it's shaped like a pool of water and looks cool. (I love him)
Lesson learned: Once mortared, don't screw with it for at least 48 hours. I'd gone out the next day because Sheri's book said to wait until the next day to put the second coat of mortar on. I sorta took that to mean I HAVE to go out the next day to put the second coat of mortar on. Like if I didn't it would cure too much and the second layer wouldn't adhere well....or something...but I should have just left him alone and let him harden more before I started messing with him.
Ah well. Lesson learned. You were right, though. This stuff gets seriously hard if you just leave it alone long enough to strengthen!