In the UK:
Reservoirs (artificial lakes) are of course owned. In most cases, there is very limited access if any for boats, and even less access for powered craft. A google search, "Boating on [name of lake]" will usually find your answer.
Canals, also artificial waterways, are managed. You can buy a day ticket, or a week, month, or year. Again, a Google search will help for the specific waters.
Rivers break down into "navigations" (where boat traffic is expected) and others. With "navigations" there is usually a licence/ticket scheme. With others, it's what you can get away with.
The coast is a mish mash of local rules. For example, for a large part of the North Wales coast, including Anglesey, you need to license your boat and show proof of insurance to launch across the beach.
Slipways are either public (free to use) or owned (in which case there may be a charge).
So, you need to consider:
The launch spot itself (free or fee?)
The water: licence or no licence?
Regardless of the local rules, it's sensible to have insurance.
A good place to find out about slipways is boatlaunch.co.uk which gives basic details for licence requirements in each launch area.
Another good place to look is
Canal and River Trust although they only manage some of the waterways.
There is a separate authority managing boating on the Norfolk Broads.
In the Lake District, you need to licence and pay for boating on Windermere, but access to many of the other lakes is unregulated — although you may need to pay if you use a slipway.
In many cases, a person using a small and unobtrusive boat, sensibly, discreetly, and not using locks, will not be asked to produce evidence that they have licensed the boat. I have only ever been asked once, which is when I was going through a manned lock.
If the water is owned and maintained by someone, then it is reasonable that there should be a charge. For example, it is only possible to boat safely on my local river, the Trent, because the level and flow are carefully controlled with a series of weirs and sluices. My local launch spot is a gated concrete slip with a car park at the top. Those facilities have to be paid for somehow.
Another reason for licensing is so they can make basic checks (self reported) on boat safety and insurance, and another is so that they have grounds to stop you, fine you, or even confiscate your boat, if you misbehave. We may be sensible boaters, but they need to allow for the worst kind of drunken yahoo who has a PWC or cheap speedboat and thinks it's funny to buzz other water users.
