Don't want to write this by hand? GodCraft sets minecraft:movement.dolphin for you with a switch.Get GodCraftOverview
minecraft:movement.dolphin is an entity component: it goes in the components object of a custom mob, or in one of its component groups. A swimming move style made for dolphins, with quick, smooth motion through water.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:movement.dolphin": {}
}