Well, Samurott can run both, but physical set for in-game, special or physical for Showdown (depending on what your team needs).
Samurott @ Life Orb
EVs: 252 Sp. Atk, 252 Speed, 4 HP
- Hydro Pump
- Ice Beam
- HP Grass
- Surf/ Scald
Samurott relies on Hidden Power as part of its moveset, so it's kinda hard to get the right type in game. If you really want to run a special set in game, go for Grass Knot. That's the only special move that gives coverage.
Samurott @ Lum Berry
EVs: 252 Atk, 252 Speed, 4 HP
- Swords Dance
- Aqua Jet
No Life Orb for this set because if you get burnt your ruined. I actually run X-Scissor on my Physical Samurott because of Megahorn's low PP & accuracy, but competitively I'd recommend Megahorn.
I would suggest a physical set over special, however, because of two reasons:
i. boosting move - Swords Dance. Samurott cannot learn Calm Mind/ Nasty Plot.
ii. +1 STAB priority, Aqua Jet. Samurott is slow even with speed EVs, so Aqua Jet is a really nice move.