Even if the daycare centre was pre-E4 where are you going to get a breed parent with Aqua Jet on it? Of the Water type within the game around five/six of them learn the move and are within Marril's egg group: Golduck, Oshawott family, Beartic* family, Seel family, Buizel family. Most comes rather late in the game and those that don't are just better than Marill in the long run. If you say I trade in a breed parent/bred Marill with Aqua Jet into your game then at that point you have to ask "Why bother breeding Marill when I could have bred a better [Xtype] Pokemon instead?". Marill family as a whole has a slow beginning, an okay middle, but starts dragging badly in the end. Edit: Late rely I know, just wanted to express my feeling on the subject.