Despite the high ceiling, I really like this stage. It reminds me of Battlefield, weirdly.
First off, there are lots of nice platforms to Nair under. You can do this not only on the middle platform, but on the two smaller side ones also, by jumping underneath, Nairing, and up-Bing to recover (sweetspotting the ledge, naturally). While your opponent will likely catch onto this, there isn't much they can do to answer your attacks except shield or move. If they're fast enough to edgehog you, you'll land in the water, which isn't too bad for G&W, you should be able to jump out and Up-B back to safety easy enough. I'm really not sure if this technique is entirely valid, the people I play with might just be tards or something
But it does work well for me when I play here.
That aside, the two smaller platforms, and to some extent the middle one also, are great for Dthrow -> Dsmash. Even nicer, the smaller side platforms are near the edge, meaning that your average Dsmash (providing your opponent is on the far left on the left platform, and far right on the right platform) will send your opponent off to the side, most likely to their doom. Also, like K2 said, techchasing with Dthrow on these side platforms is neat, and effective.
The cute and deadly klaptrap comes up every 10 seconds in the river, so watch the timer and make sure that you're not hanging off the ledge with no invincibility. Being in or near the water is kind of scary until you get used to playing on this level, so I recommend a bit of practice with it - I used to be VERY uncomfortable here, but once I got used to the river's mechanics (and watching for the klaptrap) I've been able to take advantage of this stage and all the things it offers G&W. Obviously, do not CHOOSE to go into the water. You may end up here more than you'd like to (I certainly do), but as Ampharos already said, jumping in the river intentionally is dumb.
I wouldn't take an extremely projectile happy opponent here, or rather, an opponent with long reaching projectiles (Falco, the Links, etc) although projectiles can be avoided decently, via going underneath the level (if you're not scared of the water). Still, it's not the easiest, or smartest, route to take.
The very high ceiling is kind of an annoyance, considering that it seriously impedes G&W's early kill potential. It does have a small silver lining - G&W will (vertically) live much longer than he's used to, lots of room for DI, lots of time for bucket braking. Take a character like Fox or Pikachu here, who relies heavily on vertical KOs. Or take a character with a crap (or at least below average!) recovery - the river will not be kind to them.
Has anyone ever though of taking Snake here? I have no idea how he is in the water, or how well he can camp here (I would imagine quite well). Still, his biggest threat is his Utilt, and because of the high ceiling here, it's not as dangerous. I've never played Snake here, but from what I just wrote down, I'd like to hear opinions/experiences about Snake on Japes.
Anyway, IMO Japes is a very viable CP, the only problem is that G&W has better CP's to choose from. There's little reason to choose Japes with it's high ceiling and deadly river over Green Greens, Corneria, Rainbow Cruise, Pirate Ship (!?), all safer and overall better options for G&W. 6.5/10