Because of the lack of items in RBY, Marowak cannot use his signature item, Thick Club—this severely lowers his Attack and usefulness. Marowak is a very poor man's Sandslash, lacking both in stats and movepool (namely Swords Dance and Rock Slide), which relegates him to the UU metagame, while Sandslash gets to shine with occasional appearances in OU. Marowak can get the notch in UU as the Electric attack absorber for a team, and he remains durable enough to take a few physical hits.
This is the standard Marowak set—Earthquake gets STAB and Body Slam grants possible paralysis support. Blizzard grants odd coverage for Grass- and Flying-types, especially Dragonite. Seismic Toss completes the set with its steady rate of doing 100 damage.
Focus Energy can be a good option on Marowak; however, due to an in-game glitch, Focus Energy does not work the way it was originally intended to. Therefore, Focus Energy should only be used when playing Pokémon Stadium, or in any simulator in which Focus Energy works correctly.
While Hyper Beam is ordinarily a good option on most things in RBY, Marowak doesn't really need it. Earthquake and Blizzard will usually do more damage to opposing Pokémon than Hyper Beam will. Counter has a chance to surprise opponents. Fissure makes a somewhat decent filler in an OHKO environment with paralysis support. Marowak is another of the many RBY Pokémon that work almost entirely with only 2-3 moves, and really won't make much use of its fillers anyway.
Checks and Counters
Any Water-type with respectable Defense can handle Marowak with ease, because they can drown him with their STAB Surfs. In OU that would mean something like Slowbro and in UU you can find Vaporeon and Blastoise. Despite Blizzard, Grass-types work as well, because Marowak's Special isn't worth writing home about. So in the UU environment, Tangela appears as a viable counter. Flying Pokemon not weak to Blizzard (such as Articuno and Gyarados) also stand a good chance with their immunity to Earthquake.