I've made that mistake before. Wanted to evolve two Clamperls to their two evolutions, both needing DeepSeaItems (one Tooth, one Scale). Forgot to attach, did trade, clued in on trade and was trying to fire VMs on a dying cell phone about the error. LMAO Got them done in the end, but my 3DS ended up being reset twice.
Hmm. I just realized Seadra is found on Routes 17-18, but since it appears in rippling water (likely with fishing), I never encountered it. I forgot to consider those Routes when analyzing which Water Pokémon remain to be caught. I have to reassess what I'm missing from those areas!