I think he might have the best recovery in the game, what with wall-clings, an insane up-B and Shadow Sneak. You should never not be able to make it back to the stage.
Up-throw combos into a lot of stuff if you're predicting correctly, most notably uair. Up-tilt combos into pretty much everything at varying percents, so definitely try using it whenever you get the chance. Down-tilt, while harder to hit, combos into up-smash if done right. Dash attack can combo into an aerial at high enough percentages, which means you might be able to get a kill out of it.
You can cancel the landing lag of dair into Shadow Sneak (and maybe other aerials as well, I'm not sure, dair in any case has the most ending lag, so it's most important here). This is mostly useful for mix-ups, since the ending lag of Shadow Sneak is pretty big as well. If you think your opponent will try and punish your dair, sneak in a Shadow Sneak.
The back-kick is the strongest option out of Shadow Sneak. This usually means you have to get the shadow behind them, but you can actually turn around while using the move so that you hit with the back-kick while the shadow is in front of them. You don't want to go around spamming Shadow Sneak (again, it has massive afterlag), but it can be used for approach mix-ups.
You can sometimes finish an opponent after sending them in the air by doing an up-B and using the second part of the move to go back down to the stage, if you aim the water right you can send your opponent to the upper blastzone. You can also keep characters with bad recoveries away from the edge this way. Definitely practice this move a lot.
Water Shuriken's best use is spacing. Throw out small ones to keep an opponent at a distance, if you can get a hit in you might be able to punish with a grab, or maybe even a dash-upsmash. Don't randomly start charging it because it can set you up for a punish. If you charge it, you have to be in a safe position.
His forward-smash is a good finisher, his up-smash is ridiculous. It's relative fast, has some very good range and KOs at very low percentages. I think there's a sweetspot above Greninja as well, so try hitting with that. His fair is relatively slow, but can be used as a finisher or to intercept recoveries. Don't go spamming it, it's not good enough for that.
Don't spam Substitute, it becomes very predictable and punishable. You want to use it if you know it'll work, and at higher percentages. You can angle it so your opponent gets sent up, which will allow you to follow up with an up-air or up-B, most likely KOing them.