I actually did "take" that Ape set from the analysis, but I wonder what is U-Turn used for; both Latias and Starmie outurn Ape and OHKO...
And, please, last slot for Milotic :<...
U-Turn is used to hit something as it is switching in. It is best on a choice set to scout your counter so that the next time you are in you can hit them with the best possible move.
Additionally, several of Ape's common switch-ins (cress, starmie, latias), take a huge amount from it, increasing its viability.
Either Mirror coat or Light screen is acceptable on Milotic, depending on your team's needs.
Water is one of the best offense and defense type? Yay :D...
And Poison?
Water is arguably the best type in the game. Defensively, it has 4 resists and only two weaknesses. Of those resistances, three of them are VERY good (Water, Ice, and Fire), and the fourth is quite good due to Scizor's prevalence. Of the two types that hit it, Grass is fairly uncommon.
Offensively, Water has no types naturally immune to it (though there are several abilities that give a Pokemon a water immunity) and hits almost every Steel Neutrally, making it a great coverage move. It also hits Ground types, which are commonly used Bulky walls, super effectively.
Poison is the absolute worst Offensive type, being resisted by four types, completely negated by Steels (the most common OU type), and only hitting an uncommon type (Grass) super effectively.
Defensively, Poison is quite good, however. It is arguably the best Fighting resisting type in the game, as Fighting types generally pack Ice, Rock, or Dark/Ghost type moves to hit the types that commonly resist it (Flying, Ghost, Bug, Psychic) Super-effectively. Fighting types rarely, if ever, pack a Psychic move (due to its bad coverage) or a ground move (due to its redundant coverage with Fighting). Additionally, Poison types are immune to Toxic and if not levitating/flying, remove Toxic spikes when they switch in, making them invaluable for beating certain Stall strategies.