A three-for-one special? Hmm...for now, I'll stick to the evolutions...
Huntail & Gorebyss
Shell Smash + Swift Swim will make you Speed demons. However, you don't have much on Sp.Def, nor HP. Also, Hydration gives Gorebyss a good option for defensive tactics.
Huntail excels in Attack and gets a few, yet decently powerful, Moves to show it off. Aqua Tail, Crunch, Ice Fang, Rock Tomb, Body Slam, and Super Fang (4th Gen Tutor only) make for some decent sets. However, it really needs Swift Swim active to function well. This straight-forward attitude makes it easier to figure Huntail out, but more difficult to switch-in to it, barring a K.O.
Gorebyss excels in Sp.Atk far more than Huntail does in Attack. You also have far more options to work with because of your deeper Movepool, Hydration, and buff-type Status Moves to work with Baton Pass. However, having more options gives Gorebyss less opportunity to use them. Hazers and Pseudo-Hazers will have a field day with it. This fluctuation makes it harder to figure out Gorebyss, but easier to switch-in to once its identity is discovered.
Overall, bad HP and Speed, above-average Sp.Def, good-to-great attack stats and Defense, and two weaknesses (one blockable by Ground, one uncommon outside of STAB usage) make you marginally-difficult to take down with either Pokemon. I say marginally because one good Special shot will likely be the end of Huntail or Gorebyss.
I like Water mono-Types in-general, but these two are at the pinnacle area of said mono-Type. Only Blastoise, Feraligatr, Golduck, Politoed, Wailord, Milotic, Samurott, Simipour, and the legendaries have better usage among Water mono-Types. Among Water Types in-general, though, Huntail and Gorebyss are severely outclassed, but they do manage to hold middle-ground (middle-pool?) among the full Water-Type spectrum.
A 485 Base Stat Total isn't half-bad when you consider a generous 430 of that is split between their non-HP stats. Speed suffers, but the other stats are well worth the consideration for team-building. Since Speed suffers, Trick Room is another obvious usage of Huntail and/or Gorebyss. Without Shell Smash and with Water Veil and Hydration Abilities for Huntail and Gorebyss (respectively), there are so many Pokemon that can be trolled, rolled, and rattled by one (or both) of these two in a Trick Room team...