Your question mixes two things together which are actually very different:
How efficient ... in terms of effort/time required to cover distance
Boats of all kinds are really efficient in terms of what you call "effort". You can move a lot of weight with them, while expending little energy. Speed actually reduces that efficiency, that's why the big transport ships are so slow. Human-powered boats are also slow, but they are still very efficient.
Others already gave a baseline speed for a kayak or one-seater canoe, which is around 5-7 km/h under perfect conditions (but no flow). The problem: this gives you very little information on the actual time and effort you need to get somewhere under realistic circumstances. So here a few scenarios:
Comfortable body of water. A good example for a long tour would be the TID. This is the yearly kayak tour on the Danube, from Ingolstadt and ends at the Black sea. This is 2516 km, and the time schedule for 2021 foresees 74 days for it. That's around 40 km per day, 6 days a week. This is considered a leisurely pace for a large group - a determined experienced paddler does the Rhine (Iffezheim-Rotterdam, 698 km) in 4 days of 16 to 18 hours of steady paddling, and the Unkel marathon (111 km) gets done in 8-10 hours (note that people who go on that marathon can sustain high speeds for a long time).
These times are specific to rivers similar to the Danube and Rhine - huge rivers flowing through a plain, some flow speed helping you, the beds straightened for commercial boat transports, with few sluices to carry your boat around. They are also specific to 1-seater touring kayaks. Competition kayaks, multi-seater canoes and regatta rowboats can go at higher speeds, but can only negotiate large calm rivers, small lakes and artificial channels (small ones, not the Suez!).
If you are trying to cross this type of river, you will still end up downstream from where you started, because stretches without noticeable flow are not so common. It is up to you to decide whether the effort to run back to the place where your hypothetical hiker ends up (let's say they are using a bridge) counts towards your efficiency calculation or not.
Mountains If you change the conditions, the speed will change, both in absolute terms and relative to somebody trying to cover the same ground on foot. You have a steep river like the Glenner? If you have whitewater experience and know the river, you will go down pretty quickly, and without even trying to paddle forward a lot - but you will suddenly lose a lot of time if somebody in your group swims, and you will still be beat from all the maneuvering in the whitewater. If you are doing expedition paddling though, you will spend more time scouting if the next rapid is navigable, than actually paddling. If there are places where you have to take out the boat to go around deadly rapids, you will be much slower than a hiker on a path - but if the hiker has to go around a mountain peak while you drive a shortcut through a gorge, you will still be much quicker.
If you are in a heavily populated mountain, chances are the stream won't be whitewater, but engineered to death. This will make you slower than you think, not only because of having to get out, inspect and possibly carry around at a large number of weirs, but also because of the slow backed-up water before these weirs. And hopefully, you will have bought a river guide to know the sections in which the river is guided in underwater tunnels.
Wilderness If you are on a large river system in plains outside of civilization, think something like the Okawango delta or the Amazonas, you will try to get your 4-5 km/h - but you will constantly have trouble running into thick vegetation, and if you can't go around it, you may need to backtrack a lot. Also there is unfriendly wildlife - the native people know where the hippo pools are, you might not. The hiker will be in a much worse situation than you, though.
Sea When hugging a coastline, or making small to mid-sized crossings on some archipelago, your speed will be affected by currents, tides, winds, and wave size. Unlike on a stream, you can have all of it go your way one moment and impeding you the next. I know you tried to restrict the conditions to being as benign as possible, but even on inner seas, the water is never completely calm and current-free, and tons of water, even when moving slowly, have much more strength than a tiny human in a tiny boat on top of them. You will get a bit of the same problems, especially wind, on any lake large enough to use the word "travel" with a straight face.
Note that small rowing craft is not all that unsuitable for large crossings. There are modern one-person ocean crossings (e.g. Sarah Outen), and the Vikings reached America in their ships that are about 15-30 meters long. While they did have a sail, the technology was so primitive that they had to be rowed unless the wind was coming from the perfect direction.