Yeah, it's really hard.
For one, Krabby has got a really poor Special Attack stat (50, which remains 50 when it evolves by the way). So special attacks coming from this thing do really cute damage.
On the other hand, Krabby has a really good starting Attack stat of 105, which becomes a beastly 130 upon evolution.
But. Krabby does not learn any good physical STAB moves until Level 41 (Crab Hammer). It doesn't even learn Waterfall, which is a huge shame. So it's Attack stat is never fully utilized until you are near the end of the game, which is already too late.
So in the end, I recommend you go for something else on your team. Physically Gyarados, Poliwrath and Feraligatr are nice; and Specially just about any other Water type is better (best might be Suicune and Slowbro).
Hope I helped!