Well those are relatively simple techs to correct if that's your meta. Against a water meta run the new stadium that gives you no weakness, Altar of the Sun, and/or tech in one weakness policy. Against Garb, you can afford to play a little slower and not use as many items, but you don't necessarily need to change the decklist just how you play it. Your main weakness is that this deck really needs Shaymin and if you don't have enough you're already playing from behind. In Pokémon there's quite a bit of Rock/Paper/Scissors so you will have some 20/80 matchups that are mostly unwinnable, but the key is to turn your 50/50 matchups into 70/30 and accept one or two autoloss matchups. I wish my meta was a little more Water based, I could use my Greninja deck again. Near me lots are running Grass or Metal so Fire is the counter.
It's also possible you just need to wait it out, if water is getting popular again then grass will rise to counter it, which means fire will rise to counter that, and the cycle continues.
It's also possible you just need to wait it out, if water is getting popular again then grass will rise to counter it, which means fire will rise to counter that, and the cycle continues.