Do seaplanes need to get clearance for takeoff? Do they have ATC? How do seaplanes get clearance to takeoff?
If the seaplane is operating from a land airport with a control tower AND a water runway, yes. Or, if the seaplane is operating from a Tower controlled water-only aerodrome, such as the one along the waterfront in Vancouver BC in Canada, yes also. Same radio procedures as any other airplane.
No, seaplanes generally don't need clearance to take off unless they are operating from a controlled seaplane base. There are two types of airports controlled (towered) and uncontrolled (self announce). Seaplane bases tend to be the latter and thus do not require "clearance" to take off. At an uncontrolled field pilots announce their intentions on a common frequency and depart on their own.
Everyone with a radio in an area of sufficient coverage has access to ATC but utilizing the services may be up to the pilot or may be required depending on the airspace.