Here a rate I just did for
this team.
The rate
One significant weakness I notice is a weakness to standard DDNite. Dnite can come in on any of your pokes bar Jelli locked into ice beam, DD up, and sweep. Sure, you can roar it out with Swampy, but that only delays the inevitable, as Swappy will take a massive hit before he does. To remedy this, I reccomend running a defensive Swampert instead:
Swampert @ Lefties
Nature: Relaxed, Ability: Torrent
Evs: 240 HP/216 Def/52 SDef
Stealth Rock/Earthquake/Ice Beam/Roar
This will allow Swampy to take on Dnite much better, as he will be able to take hits while retaliating with ice beam.
As for your water problem, I recommend running Ferrothorn over Forretress. Losing RS stinks for Staraptor (btw I suggestion Intimedate over Reckless for a little more survivability), but the ability to deal with water types, take special hits, and provide TWave support for Scrafty more than makes up for it. Here's the set:
Ferrothorn @ Lefties
Nature: Sassy, Ability: Iron Barbs
Evs: 252 HP/88 Def/168 SDef, IVs: 0 Spd
Spikes/Thunder Wave/Power Whip/Gyro Ball
Ferrothorn is the bain of water types existence, and fits the team surprising well. Swampy and Ferro form a nice defensive combo, as Ferro can handle the water and grass types that plague him, while Swampy can eliminate fire types that trouble Ferro. Ferro also works well with Scrafty, as his TWave support can help him outspeed threats that would otherwise outspeed and kill.
If water types still scare you, you can always run Energy Ball over Shadow Ball, though I don't recommend it.
Otherwise, it's a solid team. Gl with it! ;)
Any suggestions on how I can do better?