Yes.
During and after WW2 several aircraft were converted to serve as "Dumbo" aircraft, dropping boats or rafts near people in distress.
Other models were used as well, but the B-17 were among the most prominent.
Most were retired when the helicopter gradually took over rescue operations.
aviation.stackexchange.com/users/13485/bjelleklang
It is quite common for float planes to carry canoes & kayaks, e.g.
aviation.stackexchange.com/users/6968/jamesqf
Large enough helicopters do it.
Above example is a Boeing CH-47 Chinook ( businessinsider.com )
But since you have tagged it seaplane , not to my knowledge.
It's often the other way around, seaplane tenders tending to seaplanes (or used to).
(I take it by boat
you don't mean the inflatable and/or small type, as those are not hard to transport.)
Catalina launching beaching gear ( pinterest.com )
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I would like to suggest the "Landseaire" flying yacht which carried boats under the wings as per this question.
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It was one of the main roles for the Vickers Warwick in WWII.
From Wikipedia: From 1943, Warwicks were loaded with the 1,700 lb (770 kg)
Mk IA airborne lifeboat and used for air-sea rescue.
The lifeboat, designed by yachtsman Uffa Fox, laden with supplies and powered by two 4 hp (3.0 kW) motors, was aimed with a bombsight near to ditched air crew and dropped by parachute into the sea from an altitude of about 700 ft (210 m).[36]
Warwicks were credited with rescuing crews from Halifaxes, Lancasters, Wellingtons and B-17 Flying Fortress, and during Operation Market Garden, from Hamilcar gliders, all of which ditched in the English Channel or North Sea.[37] More on airborne lifeboats...
(also source for image above)
Of course, many WWII bombers carried inflatable dinghies, but I think the answer is looking for something more substantial.
aviation.stackexchange.com/users/4068/Brian Drummond
The Fernic T-9 of 1929 was prepared for an Atlantic crossing and as a precaution had removable upper engine nacelles which could double as a life raft, including an outboard engine.
A less planned use of an airplane part as a boat occurred when in 1932 the Junkers W-33 of Hans Bertram and Adolf Klausmann crashed in a remote part of Western Australia.
They removed one of the floats and made it into a boat for fishing and excursions.
aviation.stackexchange.com/users/1961/Peter Kämpf
Took me a while to find this photo, from my personal archive from the East Fortune Airshow 2015 of a Royal Norwegian Airforce Lockheed Orion.
As I recall, the commentator stated that the orange object visible in the 'bomb bay' is an air-droppable lifeboat, for their search-and-rescue missions.
aviation.stackexchange.com/users/40162/Mike Brockington
The Boeing C-17 Globemaster III can deploy boats.
There are some videos on YouTube ( example ) that show the process:
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A small boat could be part of general cargo, but that would typically just mean it's unloaded at an airport and transported to the nearest body of water by land infrastructure. This is not what I'm looking for.  Instead, is there any aircraft that can directly deploy a boat to (or retrieve one from) water?  I would imagine there are cases when moving a boat by plane could be useful, especially when there's little or no infrastructure available on land.  Does such an aircraft exist?
Do any aircraft carry boats?
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<cargo><seaplane>
aviation.stackexchange.com/questions/68497
aviation.stackexchange.com/users/42832/SE - stop firing the good guys
During WW2 many planes carried boats for various purposes. The Boeing C-17 can also deploy boats. Moreover, it is quite common for float planes to carry canoes and kayaks. Finally, large helicopters also can carry boats. There are aircraft that can carry boats; at the time of the 2nd World War many models did it, mainly B-17 or the  Vickers Warwick. The "Landseaire"  also served that purpose and even the Boeing C-17 Globemaster III can deploy boats. Also, float planes often carry small boats like canoes, etc. It must be noted that large helicopters are used for this task as well.
There are aircraft that can carry boats; at the time of the 2nd World War many models did it, mainly B-17 or the  Vickers Warwick. The "Landseaire"  also served that purpose and even the Boeing C-17 Globemaster III can deploy boats. Float planes often carry small boats like canoes, etc. Large helicopters are used for this task.