KO'd a friend from 40 life with Vicious Shadows + Blasphemous Act with a board full of his creatures. The best part was getting this off at 2 life after being pummeled all game.
Iona is the only moderately useful thing I can come up with at the moment. I suppose there's also Elderscale Wurm and Pelakka Wurm if you just want a big beater for free.
I don't think Gamekeeper has been mentioned. Also worth noting is Pattern of Rebirth, which can give any creature a ETG effect and combos well with self sac effects.
I also run a Jenara counter themed deck, and so far it's a blast to pilot.
Vigor - Protects your guys, makes them bigger. Death's Presence - A bit weaker, admittedly, but can give you some spot removal protection. Plaxcaster Frogling - I know you said Simic aside, but this guy can be an all star, so I wanted to mention him by name. Doubling Season - Obnoxious with almost all interactions in the deck. Venser, the Sojurner - Can bounce for evolve triggers, proliferate enablers, make your guys unblockable, etc.
Kederekt Leviathan - Doubles as mass bounce, recurrable from gy.
Lorthos, the Tidemaker and his little brother Shipbreaker Kraken can help lock down a troublesome board.
Colossal Whale and to a lesser extent Giant Oyster can be on-theme removal.
Stormtide Leviathan keeps up with the Islandwalk theme.
Trench Gorger big dumb beater.
In addition, if you end up favoring a more Merfolk heavy build, don't forget anthem effects.
Master of the Pearl Trident
Lord of Atlantis
Caged Sun
Gauntlet of Power
Finally, High Tide is a must. Because, y'know, flavor.
I've also had fun with Hesitation, interacts well with Sun Titan.
Also:
Decree of Silence
Standstill
Teferi's Moat
Reparations
As far as wincons, Sigarda, Host of Herons, Simic Sky Swallower, and Stormtide Leviathan all provide reasonable protection with decent bodies attached.
Vigor - Protects your guys, makes them bigger.
Death's Presence - A bit weaker, admittedly, but can give you some spot removal protection.
Plaxcaster Frogling - I know you said Simic aside, but this guy can be an all star, so I wanted to mention him by name.
Doubling Season - Obnoxious with almost all interactions in the deck.
Venser, the Sojurner - Can bounce for evolve triggers, proliferate enablers, make your guys unblockable, etc.