Don't want to write this by hand? GodCraft sets minecraft:floats_in_liquid for you with a switch.Get GodCraftOverview
minecraft:floats_in_liquid is an entity component: it goes in the components object of a custom mob, or in one of its component groups. Makes the entity float up in water and lava instead of sinking.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:floats_in_liquid": {}
}