Moves
While not being a STAB move, Thunderbolt is the most important move on the set, as it threatens opposing Water-types, which this set is meant to reliably answer. Notably, it manages to KO Floatzel after two rounds of Stealth Rock, as well as 2HKOing physically defensive Mareanie and OHKOing Swanna. The second slot is reserved for another attacking move: Ice Beam is most common, as it synergizes very well with Thunderbolt, heavily threatening Altaria, Gourgeist-XL, Leafeon, and Golem. Surf is Silvally-Water's most powerful move due to STAB and does considerable damage to most Pokemon that don't resist it, but it leaves Silvally-Water walled by most Grass-types. Hazard control is essential on every team and Silvally-Water is a good user of Defog thanks to its good matchup with many of the metagame's entry hazard setters like Golem, Probopass, and Crustle. U-turn allows Silvally-Water to pivot out of bad matchups after removing hazards or keep up momentum on forced switches. It also does decent damage to Pokemon weak to it like Servine and Exeggutor thanks to Silvally-Water's solid base Attack, which means that Ice Beam is not as important as it otherwise would be. Parting Shot is an option over U-turn to offer utility in stat drops, but it can be blocked by Soundproof Bouffalant and Mr. Mime and gives Defiant Pawniard and Purugly +3 Attack, making U-turn much safer.
Set Details
160 Speed EVs and a Timid nature allow Silvally-Water to outspeed Servine, meaning it can U-turn out on it or threaten it with Ice Beam while also moving before Pokemon like Altaria and Monferno. 100 Special Attack EVs ensure that Floatzel goes down to Thunderbolt after two rounds of Stealth Rock damage, allow Silvally-Water to 2HKO Mareanie with Thunderbolt, and enable it to OHKO physically defensive Altaria with Ice Beam after Stealth Rock.
Usage Tips
Silvally-Water should be focused on pivoting in and out, constantly keeping up momentum. In order to do this, it needs to ensure entry hazards are getting consistently removed, and it needs to avoid status, as it will get chipped too quickly otherwise. It has solid natural bulk, and this is bolstered by HP investment, which lets it switch in once or twice against certain attackers of which it does not resist the STAB moves, like Pinsir. This can be very useful to immediately remove hazards, threaten the opposing team, or generate momentum. If there is a Pokemon on the opposing team that Silvally-Water needs to check, like Floatzel or Swanna, it is important to keep it healthy to neutralize that threat. Remember that Silvally-Water does not have any way of getting its health back; while it wants to pivot in and out, it's important to carefully consider what damage is worth taking. Due to Silvally-Water's wide movepool, your opponent will often be unsure of exactly what moves Silvally-Water has, and you can use this to your advantage. By spending much of the early-game using U-turn to pivot in and out and not revealing Silvally-Water's set, you can lull your opponent into a false sense of security before suddenly catching them off guard with a coverage move. Silvally-Water can be used as a decent Knock Off absorber, as RKS System prevents Water Memory from being removed and prevents the move's power increase, allowing it to switch into Pokemon like Pawniard effectively.
Team Options
Especially if you're running Surf, a good answer to offensive Grass-types such as Torterra and Leafeon is necessary. Defensively, Steel-types like Bronzor and Mawile are the best, and offensive answers include Simisear and Rapidash. Bronzor and Mawile also form a good entry hazard control core with Silvally-Water. Since Silvally-Water is a solid answer to Water-types, Pokemon that hate their presence appreciate the support it offers. Thus, the previously mentioned Simisear and Rapidash but also Pokemon like Choice Band Bouffalant enjoy being paired with Silvally-Water. In return, each of them threatens Grass-types. Since Silvally-Water does not have any form of recovery, it can easily be crippled by status, most notably Toxic. Thus, a status absorber like Komala is quite useful. Note that Komala can also heal Silvally-Water back up with Wish, adding to its longevity. A Pokemon immune to Electric-type moves, like Electivire or Sandslash, is important as a teammate, as Silvally-Water is easily forced out by Volt Switch users like opposing Electivire. Silvally-Water fits well onto Fire / Water / Grass cores, so partners like Gourgeist-XL, Leafeon, Rapidash, and Combusken can form a deadly combination.