Just a guess but they probably felt that the bottom needed the ribs but topsides were adequate. Save lumber and lofting time, in any case, it works...:)
Don't dis yourself, I think a previous owner changed the cable and ran it in to tight and cracked the darn thing. Look at a manual and confirm that thought...
I assume you have a fresh water holding tank so you must have a fill cap for it, the water heater is also connected to the fresh water tank hopefully..
I could be wrong, probably am, but that doesn't look like a fresh break.. it may have been cracked for a while.. fixing it is beyond me but thanks for the picture and it may help...
That looks like no pick-up tube I've ever seen.. looks like a pipe plug. In any case hold the large fitting with an open end wrench and then remove the small square fitting..