Mono-Water's Answer to Mega Venusaur?

You could possibly try giving Starmie a shot. Since most Water types are doomed against Mega Venusaur's Giga Drain, you could take some luck into your hands and hope a solid Psychic can KO Mega Venusaur before it even gets the chance. The reason I chose Psychic over Psyshock is because most defensive Mega Venusaurs are Physically defensive. Psychic also has 10 more base power, so its a better gamble for an OHKO with it.

Here is the set:

Starmie @ Choice Specs
Ability: Analytic
EVs: 252 SpA / 4 SpD / 252 Spe
Modest Nature
- Psychic
- Hydro Pump
- Thunderbolt
- Ice Beam

Against a specially offensive Mega Venusaur...
252+ SpA Choice Specs Starmie Psychic vs. 232 HP / 4 SpD Mega Venusaur: 344-408 (95.8 - 113.6%) -- 75% chance to OHKO

Against a physically defensive Mega Venusaur...
252+ SpA Choice Specs Starmie Psychic vs. 252 HP / 0 SpD Mega Venusaur: 344-408 (94.5 - 112%) -- 68.8% chance to OHKO

Its risky, but its pretty effective if luck is on your side. Starmie also helps with coverage against other water types and if its hit with a priority move or is outsped, Analytic will give an extra power boost. With 115 speed, losing the speed investment for a special attack boost helps psychic have a higher chance of OHKO'ing Mega Venusaur.

 

I guess Tentacruel would help when against it, due to most Mega Venusaurs using Giga Drain. Tentacruel's ability, Liquid Ooze can help you take down Mega Venusaur, since it makes Giga Drain and the occasional Leech Seed, decrease health instead of recovering it.

This is a example set.
Tentacruel @ Black Sludge
Ability: Liquid Ooze
EVs: 248 HP / 204 Def / 56 Spe
Bold Nature
- Scald
- Acid Spray / Sludge Bomb / Knock Off
- Rapid Spin
- Toxic / Toxic Spikes / Knock Off

Other option could be to use a Gyarados with Bounce or Mega Gyarados with Ice Fang.

Bulky DD Gyarados should work quite well, being able to block Leech Seed and other potential Status moves with either Taunt or Substitute.


Gyarados @ Leftovers
Ability: Intimidate
EVs: 88 HP / 192 Atk / 4 Def / 224 Spe
Adamant Nature
- Substitute / Taunt
- Dragon Dance
- Waterfall
- Bounce


For Mega Gyarados I recommend using this set.

Gyarados @ Gyaradosite
Ability: Intimidate
EVs: 252 Atk / 4 Def / 252 Spe
Adamant Nature
- Dragon Dance
- Waterfall
- Earthquake
- Ice Fang

 

I think a good choice would be Ludicolo
Being part Grass-type, it doesn't take super-effective damage from Giga Drain, and it's immune to Leech Seed.
Granted, it takes super-effective damage from Sludge Bomb, but pairing it up with Rain Dish and Leftovers should fix this problem (assuming you're running Rain Dance on an all-water team)

Ludicolo @ Leftovers
Ability: Rain Dish
EVs: 252 HP / 4 SpA / 252 SpD
Modest Nature
- Fake Out
- Ice Beam
- Giga Drain
- Scald

Using Fake Out when Ludicolo is first sent out will get rid of a little HP from M-Venusaur. Then just spam with Ice Beam, or try and get a burn off with Scald.

 

Speed and Sp.Attack invested Starmie or a Speed and Phys.Attack invested Cloyster should solve all your troubles.

 

While Starmie could help, dealing decent amount of Damage with Psychic and being able to OHKO Offensive Mega Venusaur with if there are 3 layers of Spikes or 1 layer of Spikes and Stealth Rock on the opponents side of Field. If it's defensive you need to have 3 layers of Spikes or 2 layers of Spikes and Stealth Rock. to have 100% chance to OHKO.

However, with Cloyster, you need to hit Mega Venusaur on the switch in while you have either Stealth Rocks or 1 layer of Spikes on the field. That is for the offensive variant. Keep in mind that you need to 1 layer of Spikes and Stealth Rocks on the field to OHKO with +2 Adamant Icicle Spear.
For the defensive variant, you need +2 Adamant Icicle Spear along with Stealth Rocks and 3 Layers of Spikes to OHKO it for 100%.
It's also good to keep in mind that Mega Venusaur has the option to outspeed Cloyster, though it's not very common option due to lower bulk and it's not needed usually in OU.

 

Yeah Mega Venusaur is just too great I guess you'll have to give up.... ha ha just kidding. After reading what's above paired with my Venusaur experience I'd say your best options are the offensive starmie set and the tentacruel. I don't know the rest of your team but if I had to make a choice I'd say tentacruel because I love poison types and that one has massive staying power, plus starmie can't get sucker punched or shadow sneaked. Ludicolo would work in some scenarios, and I can see empoleon with possible success but it'd be less than ideal.

But do you want something original because I have it.

What's the most deadly element in a battle? Not fire, not water, it's surprise. And this set will open some eyes.

Pelipper @ Choice Specs
Ability: Rain Dish
EVs: 4HP/ 252 SpA/ 252 Spe
Modest Nature
-Hurricane
-Hydro Pump
-Tailwind
-??????? (endless possibilities, sky drop, HP dragon, knock off)

Now hold on before you start booing and listen to the cold hard facts. How many people are ready for an offensive pelipper? None. Venusaurs will be like "Ah fam I got this big faced bird don't worry" Let the numbers decide for you.

252+ SpA Choice Specs Pelipper Hurricane vs. 252 HP / 0 SpD Mega Venusaur: 378-446 (103.8 - 122.5%) -- guaranteed OHKO

Now look at that closely. Aint no Pokémon safe from this bird. Tailwind is to support the rest of the team that is lagging behind your massive wingspan. And the last move is where it gets real. Sky drop is for mind games, no way is anyone ready for their beloved poke to get picked up and destroyed by your death bird. Or HP dragon, because on the inside pelipper is secretly a dragon and when it gets a mega it will be the first 3 typed Pokémon with water/flying/dragon so you'll thank me later. Knock off is just if you're a hater who has no friends.

If you got the rain coming down you get that clean rain dish health and them toasty accurate hurricanes. We have a clear winner here.

 

Go with Turnip's Tentacruel set but Mirror Coat can hit a ton of things in the Metagame such as Hoopa-U, Celebi and ANY special attacker!