Ah, relax, 'mon. The Humilau Gym is about as relaxing of a Gym as could be. Talk to Clyde and he'll give you a Fresh Water, which is great, because the water around these parts is rather salty.
Just up a bit and to the left, you'll see a lily pad. Hopping on the lily pad causes it swim off (with you on top of it) over in the direction you're facing, until it hits something. Go ahead and hop on it to reach the side dock, then fight the Ace Trainer up there. He uses a Floatzel L47 and a Mantine L47. You've gotta watch out for the likes of Mantine, because it is part Flying-type (rendering Grass-type Pokemon ineffective against it), plus it has a ton of Special Defense.
After beating the Ace Trainer, you'll be able to see another interesting dynamic of the lily pad puzzle by hopping onto the lily pad next to him. It'll bump into another lily pad, stopping yours but pushing the other one forward. You'll have to keep that in mind while solving the puzzle.
Now, head up and jump onto the lily pad. You'll have to fight the Ace Trainer just above you. She's got an Alomomola L48 to bother you with. After taking her down, head south onto the lily pad to reach another dock area. You can now cross the lily pad you pushed while getting to the last Ace Trainer.
Follow the path to the north (you can leave to the south if you need to heal) and cross the bridge. Keep following the path until you see a lily pad in the corner. You can approach it either from the north or from the west, pushing it south and east respectively. Go ahead and push it south, then get off and push it off to the east. You can fight the Ace Trainer up there (you're not forced to, though). He uses a Walrein L48, which may also pose a problem for your Grass-type Pokemon.
Now go back across the bridge and head north. Fight the Ace Trainer, using her Pelipper L47 and Wailord L47. You can ride the lily pad either north or to the west. Riding it north won't take you anywhere, so head west on it. There's yet another lily pad. Go ahead and ride it north, up to another Ace Trainer. She uses a Golduck L47 and a Starmie L47.
Ride the lily pad south and fight another Ace Trainer. He's got a Basculin L46, a Basculin L46, and lastly an Azumarill L46.
Ok, time for the fun part. Ride back to the west, then circle around and ride the lily pad north. Now disembark and ride it to the east. You'll bump a lily pad off to the side. Once you've done that, you'll be right where the Gym Leader, Marlon, is hangin' out! If you need to heal, just ride the lily pad to the south to exit. You can hop right back on it after leaving to get back to him easily, so feel free to heal if you need to. Otherwise, get ready to fight the final Gym Leader in the game!
His Carracosta likes to start the fight off right by using Shell Smash, which lowers both of its defensive stats by one stage, but then raises its Attack, Special Attack, and Speed by a staggering two stages! That can turn it into a monster real fast. It has Sturdy, too, so you can't even take it out in one hit. Grass-type attacks are most effective against it, but a solid Electric-, Fighting-, or, if you're brave, Ground-type attack will work, too.
While his Wailord may look impressive in size, it's not too tough, due to its low defensive stats. It may burn you with Scald if you aren't careful, though.
Last up is his Jellicent. This Water/Ghost-type may be a bit of a hassle to take down, but after beating Drayden's Haxorus, it should be a walk in the park. Try to avoid letting your HP stay below 50%, though, because it packs Brine, which will do double damage if your HP is below 50%. It, too, packs Scald to potentially burn you.
After beating him, he'll reward you with the final badge to fill your case: the Wave Badge! This ensures all traded Pokemon obey you without any problems. He'll also give you TM55 (Scald), a fairly decent Water-type attack (80 power) that has a 30% chance of burning the foe. Surf is usually a better option, but sometimes the single targeting of Scald or the burning effect can be handy. Once you've gotten your loot, he'll jump off into the water. See ya!
Head south on the lily pad to take a nice, relaxing ride down to the entrance of the Gym.
As soon as you leave, Hugh will congratulate you on getting your Gym Badge. Marlon will show up, though, and tell you he saw some Team Plasma guys. He then asks if they're bad people. Go ahead and say yeah. He then heads off to go find them or something. The two of them will now head south, but you're not going to be able to get too far until you visit Route 22, so head on northwest to Route 22!