mono u fish for modern
running 20 or 21 land should also work depends how confidant you feel in your aethervial drawing abilities.
consider adding 3x thassa (replacing 1 phantasmal image, 1 kira and 1 cursecatcher)
despite having the deck making high devotion most of the time master of the waves is a bad card for merfolk decks due to his high cmc and the fact that he makes and buffs elementals instead of merfolk. - do not add.
for budget:replace mutavault with more islands/faerie concalvereplace remand with mana leakreplace cursecatcher with spellpierce or spellsnare
aethervial is very important to the deck and should be your first priority budgetwise but could be replaced with any of the above mentioned counterspells.
coralhelm commander or merlfok sovereign can replace any unavailable lords
sideboard:this depends alot on your local meta i have included a fairly broad sideboard for use against storm/pod/control & graveyard shenanigans.the mainboard should be fine as is against affinity or jund however if you have a problem with these consider addinghurkyls recall for affinitylightning greaves if they use lots of spot removaltrickbind if they use alot of fetchlands (also good against pod)ghost quarter if they dont use any/many basic landsvaporsnag if they have a very agro deck (also good against zoo)surgical extraction is good against combo decks like twin (however torpor orb will do this job just as well)
if you want to splash a second colour:3/4 scalding tarn/ misty rainforest3/4 shocklands of the relevant colour2/3 checklands of the relevant colour1/2 basics of the relevant colour
for white (would recomend splashing white if budget allows)add path of exile to mainboardadd kataki, meddling mage and Sejiri Merfolk to sideboardconsider day of judgement/ supreme verdict
for blackadd thought seize and dismember/doomblade/ go for the throator lilliana of the veil to mainboardadd leyline of the void and notion theif sideboardconsider bloodcheif ascension and deathrite shaman
for greenadd beastmaster acension and simic charm to mainboardadd krosangrip / arifact & enchantment removal, trygon predator to sideboardconsider overrun / overwhelming stampede type effect.consider deathrite shaman
for redadd lightning bolt and izzet charm to mainboardadd artifact removal to sidebaordconsider bonfire of the damned, chandra nalaar and browbeat.
