While it doesn't have synergy with Delver, I find Engineered Explosives to be the best sweeper against Merfolk. EE on 2 hits the majority of their threats and blows up any Spreading Seas that might be hampering your color production. I see that you have one in your board already. If you really fear the Fish, I'd up that to two. I'm not really sure where Merfolk is at in the meta at the moment. The shift to UG seems to have sparked some revived interest in the archetype, but it's kind of pricey for a tier 2 deck, so that might be keeping people off of it.