This deck was not great, but I could win about half the time with it at low gold and fulfill quests that involved casting U or G spells.
Raphael Levy (Hall of Famer) took this to the last Standard Mythic Championship and did quite well.
In your shoes I would start with Kumena, Deeproot Elite, however many Breeding Pool, Hinterland Harbor, Unclaimed Territory, Silvergill Adept, Deeproot Waters and Merfolk Mistbinder as you can muster. Add the Skydivers, Jade Bearers and Herald of Secret Streams along with any other merfolk at less than 3 mana that you want to try.
Spend some uncommon wildcards on up to 3 of the good uncommons (Mistbinder, Silvergill Adept, Deeproot Waters, Unclaimed Territory). Maybe throw in a few spells that you like (Contentious Plan?, Incubation, // Incongruity, Spell Pierce, Dive Down, Disperse).
Fill in the lands with some Simic Guildgates and Woodland Stream, Islands and Forests until you have at least 14 G sources and 14 U sources and about 23 lands.
Fill in up to 60 with merfolk around the 1-3 mana cost so that you don't have too many 3 drops.
Test and iterate.
When Dominaria comes round as the draft format (hint: the day after tomorrow) do as many drafts as you can and snag Hinterland Harbors if possible. Likewise Ravnica Allegiance for Breeding Pool and Benthic Biomancer. Ixalan block for lots of cool merfolk of all rarities and Unclaimed Territory.
I hope this helps, but I realize I may have gone far afield from your intention. Please feel invited to correct my misapprehension or post your next iteration.
If you can, the decktags are super-easy and make it easy to mouse over your deck and see what the cards are. They are described here.