Eldritch Blast is ranged and shoots two blasts (5th level spellcaster). Range 120ft and forcing two Concentration saving throws. You can add Hex for extra ranged damage.
Also, which water cultists don't have a ranged weapon? (Can't check the module at the moment.) Reavers have javelins... priests have spells. Even the fathomer inside silence could still throw his daggers in hope to break concentration.
(Note: I usually assume that monsters usually have 2 or 3 pieces of thrown weapons on them, at least if they're on guard duty. Also, some enemies might just exchange their weapons. Recently, I had a spellcaster give his dagger to an ally that lost his weapon for example.)
To answer the question of why the spellcaster doesn't walk out of the area of silence - they often do, but the PCs have high DEX so often come in high on the initiative order and so the spellcaster can easily have taken multiple arrows/other damage before they even get a chance to move.
I honestly don't understand this issue. Even assuming all PCs roll above the enemies (very unlikely even if they have higher DEX), setting up Spike Growth and Silence already takes a turn, so at tops you have 3 PCs attacking before the enemy can start taking countermeasures. Also this has nothing to do with those two spells in the first place. If your PCs can already do such significant damage in a single round that the monsters don't stand a chance, the PCs might just be too high level for the encounter.
Of course if the encounter is written with reinforments in mind which don't come because the Gong is silenced, that one encounter might be easy. And I'd grant my players that for the clever idea to silence the Gong. But eventually the enemies will know they are under attack and start to gather before the PCs even arrive, forcing them into hard or deadly encounters.
