From my experience, Relicanth makes a pretty damn good Rain-setup pokemon. Unlike its competition in Lanturn, Lapras etc, it's VERY physically defensive and can hit nuisances like Shedinja with Stone Edge, whilst also providing Stealth Rock support. I've used something like a 252 Atk / 252 Spe Adamant spread with Stealth Rock / Rain Dance / Waterfall / Stone Edge and Swift Swim. If you'd prefer to use Head Smash and are willing to sacrifice utility against offensive pokemon, use Rock Head and more defensive EVs.
I like the set you provided, but I personally find that Relicanth doesn't hit hard enough for the *usually* offensive nature of Swift Swim without at least a Life Orb and optionally Head Smash.
On the set up aspect, Water typing and a 4 times Grass weakness isn't good for a rain setter. Rain Dance teams often rely upon the setters for their ability to tank certain types of moves such as Grass and Electric attacks. The fact that Relicanth doesn't have access to U-turn or Volt Switch is not helpful either.
I have been testing a Rain Dance set-up / sweeper combination set lately:
name: Rain Dance
move 1: Rain Dance
move 2: Waterfall
move 3: Head Smash
move 4: Earthquake
item: Life Orb
ability: Swift swim
nature: Jolly
evs: 252 Atk / 4 Def / 252 Spe
It works by breaking down the counters a team has to water type attacks. When the opponent switches in the bulky water type or whatever you demolish it with Head Smash. Not many NU pokemon can take a combined Rain Dance boosted Life Orb STAB Waterfall and STAB Head Smash (or many pokemon at all for that matter).
I also find that even after two attacks worth of Life Orb recoil and the Head Smash recoil you still usually have enough HP and speed to hit one more pokemon. Jolly 2+ Relicanth outspeeds 1+ Rotom-A and below too so there is not much that can revenge kill it in NU either.
Having said that you can easily combine the two sets:
name: Rain Dance
move 1: Rain Dance
move 2: Waterfall
move 3: Head Smash / Stone Edge
move 4: Earthquake / Stealth Rock
item: Life Orb / Damp Rock / Leftovers
ability: Swift swim
nature: Jolly
evs: 252 Atk / 4 Def / 252 Spe
But the Slashitis is bad =0.
Perhaps I could just mention Rain Dance over Rock Polish in the AC of that set.