Recent changes to entity mounting allow dolphins to be ridden underwater, but when a dolphin is given or summoned with a living passenger (other than a player) and enters water, its speed increases to ranges well in excess of 30 meters per second.

This can be demonstrated either by spawning a mob and mounting it on a dolphin with /ride, or summoning the stack directly and dropping it in the ocean.

/summon minecraft:dolphin ~ ~ ~ {Passengers:[{id:"drowned"}]}

Non-living passengers such as armor stands or interaction entities actually seem to cause the opposite effect, bringing the dolphin to a dead stop.

/summon minecraft:dolphin ~ ~ ~ {Passengers:[{id:"armor_stand"}]}

Players riding dolphins do not seem to provoke this behavior, or at least not consistently.