First, Clamperl is one of the worst water Pokemon in Emerald because it comes late and needs trading to evolve. One thing you can do, given that your Tentacool is level 13, is finding a higher level Tentacool or Wingull or using a super rod to fish for a Sharpedo. All 3 of these Pokemon have much better stats compared to Clamperl.
If you absolutely must use Clamperl for whatever reason, the better item is usually deep sea tooth. Let's assume the average late-game opponent has 80 base stats, 80 base power STAB moves, and no type advantage or disadvantage against Clamperl. To do damage calculations with this theoretical Pokemon, I put a ghost dark type Glalie with shadow ball and crunch in the damage calculator.
Deep sea scale calculations:
0 Atk Glalie Shadow Ball vs. 84 HP / 84 Def Clamperl: 43-51 (38 - 45.1%) -- guaranteed 3HKO
84 SpA Clamperl Surf vs. 0 HP / 0 SpD Glalie: 53-63 (34.1 - 40.6%) -- guaranteed 3HKO
Deep sea tooth calculations:
0 SpA Glalie Crunch vs. 84 HP / 84 SpD Clamperl: 59-70 (52.2 - 61.9%) -- guaranteed 2HKO
84 SpA Deep Sea Tooth Clamperl Surf vs. 0 HP / 0 SpD Glalie: 105-124 (67.7 - 80%) -- guaranteed 2HKO
source
According to these calculations, the deep sea tooth allows Clamperl to do a lot more damage while taking only a little more damage from the Glalie.
