Drop Swampert. Its Speed is atrocious, and while it does have the benefit of immunity to Electric-types, that still doesn't make up for its poor stats.
A good Water-type to use is Lanturn. It has decent stats (still doesn't have very good Speed, but it is faster than Swampert), and its dual-type nature really helps a lot. You'll now have two basics covered (Water and Electric) in one Pokemon. Teach it Rain Dance and Thunder from TM's, and you'll have one of the best move combo's around (if you didn't know, using Rain Dance insures that Thunder, a move with normally bad accuracy, will never miss). Lanturn also learns Hydro Pump, which gives you some good firepower for Water-type moves. Alternatively, you can teach it Surf instead, if you don't like Hydro Pump's lack of accuracy.
Next, I suggest an Ice-type, since you never know when someone will throw a Dragon-type at you out of nowhere. Glalie and Walrein are really your only choices in R/S/E; of the two, I'd take Walrein. Better stats (except Speed), and better moves. Glalie is faster, but its other stats are all lower than Walrein's.
Thirdly, if you can, a Dragon-type might be something to consider as well. That would mean dropping Skarmory (which really isn't a bad Pokemon at all, just a victim of a weak movepool). It's not really neccessary you do this, but Dragon-types have so many immunities to so many different types, so I'm recommending you try at least. That fact alone will really help you, even given the lack of truely powerful Dragon-type moves (or Pokemon, for that matter). The best choice is Salamance, so find a Bagon in Meteor Falls, and train the heck out of it. You'll have the benefit of Dragon's resistance to most types, and Salamance has the benefit of learning a variety of moves through TM's, including Fire-type, which eliminates the lack of any Fire-based moves on your team right now.
That has most of the basics covered. Some things you still might struggle against, like Psychic and Dark Pokemon, but it isn't a weakness; you just don't have anything that is extremely effective against those. Having Water, Electric, Ice, Dragon, and Fire type moves helps a lot against a variety of types. Walrein can also learn Earthquake, giving you a Ground-type move also, making it a total of six different types you can use in only three Pokemon.
Final Team:
Lanturn
Walrein
Salamance