These playful, bottlenosed "friends" of humanoids everywhere constantly follow ships and look for those shipwrecked. Not to help them of course, but because they are an easy meal.
(This is part of my ongoing conversion of Castle Greyhawk monsters.)
Medium 4th level troop [beast]
Flipper Splash +7 vs PD, harmless (no damage)
Bloodletting Rend +9 vs AC (any creature in the water) - 14 damage,
Natural 16+: Target bleeds for 5 ongoing, and is now Vulnerable to other killer dolphins.