Don't forget about
Cursecatcher. I see a lot of new Merfolk players make this mistake. Always try to counter a sweeper, even if they can pay for it, since they might have a follow up play.
You can
Vapor Snag your own creatures. I do this a lot to protect
Master of Waves but I've also used it to protect lords when it's important to do so.
Try to get 2 lords as quickly as possible out against Tron.
Pyroclasm will be blanked.
You don't have to tell your opponent how big your creatures are. A lot of playing Merfolk is about doing combat math, and telling your opponent exactly what's going on is disadvantagous. Obviously don't be a dick about it, this only applies to high levels of competition. If they're just taking the damage (i.e. asking "how much is that"), make sure to ask if they're declaring any blockers first.
On the other hand, you should know exactly how big your creatures are.
Silvergill Adept's,
Mutavault's, and
Master of Waves's power is 2 + the number of lords, and the lords and
Cursecatcher's power is 1 + the number of lords.
Mutavault also gets an extra boost from
Master of WavesCut
Aether Vial against Jund or any other attrition-based deck (i.e. Grixis control). A lot of Merfolk players tend to leave it in, but if the deck is just seeking to one-for-one you, you could have 4 threats in the place of Vials.
In the mirror,
Lord of Atlantis gives any opposing Merfolk +1/+1 and islandwalk as well.
Master of the Pearl Trident doesn't. It's always safer to cast
Master of the Pearl Trident first if you're unsure about what your opponent is playing.
If you
Spreading Seas a manland, your opponent can activate it in response, and it will remain animated until the end of the turn.
Keep
Mutavault +
Aether Vial and no other lands game one. It isn't safe to do so game 2 or 3, since your opponent might have brought in artifact hate. Generally don't keep one land hands unless you have
Aether Vial since most of the threats in the deck cost 2.
With
Wanderwine Hub and
Silvergill Adept, try to reveal your play for the turn, or what you will be playing the turn after. Try to conceal
Harbinger of the Tides and
Cursecatcher if you can flash it in off
Aether Vial