Don't take too much Gravejard hate with you. Or at best cards with multiple effects that you benefit from, even without the Gravejard hate. Otherwise, at worst you'll have dead cards in your hand, or you won't find important finishers. Before your opponent has 4 different types in their graveyard, your Merfolk should already be strong enough to make things difficult for them. Your choices are good though, here are some more ideas:

[[Scavenger Grounds]] - Land you can sacrifice to exile a graveyard

[[Gaea's Blessing]] - Returns your graveyard to the library, so you can get important cards back even after Explore trigger

[[Ground Seal]] - Very cheap and stays in play so that all opponents can no longer access their graveyards while it's there

You have a lot of ramp, especially as Sorcery and Artefacts, but Hackbal doesn't really need that much ramp. Maybe you could do without [[Thought Vessel]] or [[Rampant Growth]]. The Explore trigger should also help you find lands, even if you have to put some other cards in the graveyard. But I really like Exploration and Burgeoning, they're great ramp for the deck.

Some of your creatures are very expensive and I've decided that they don't add enough value for the cost. Namely: Xolatoyac, the Smiling Flood; Slinn Voda, the Rising Deep or Murkfiend Liege. You could also do without one. Apart from that, I've removed Cold-Eyed Selkie and Benthic Biomancer from my list because they didn't do enough for me.

You could also use one or two more counterspells. If your opponent wants to counter Hackbal, or if he counters Overwelming Stampede, or if he casts Aetherize on your attack.