Sample Decklist Bria, Riptide Rogue gives prowess to all of your creatures, so you'll want both a lot of creatures to capitalize on prowess and a lot of cheap, efficient spells to trigger it frequently
Thrummingbird Thrummingbird is cheap to play, so it doesn't come with much punching power
The value it generates as a 1-mana loot effect, drawing you two cards and letting you pitch two essential graveyard cards, with the option to flashback, is frankly insane
Its a Commander with a ton of value (Im looking to squeeze him into another underwater-themed deck as we speak), but theres also Kindred Discovery in the deck as well to get even more card draw from your Merfolk
For the 99, the classic pingers still earn their slots: Guttersnipe and Electrostatic Field
Grave Titan offers a good amount of defense, a good amount of value and a very snowbally win condition that can attack while retaining defenses