Here are some low cost water heaters. These are based on decades of my own experience and experimentation. Trickling small amounts of cold water through these solar-collectors can gradually fill a hot water tank. In the simplest system, heated water can be used directly. Where plumbing codes require, use of garden-hose fittings (instead of permanent fittings), might provide compliance.
Picture above is an example of very simple solar water heating, positioned on a roof. For cooler climates, it is enclosed in a plastic envelope to greatly increase heating. Widely available, black PE plastic tubing is generally the lowest cost tubing or pipe. It comes in 100 ft rolls, a random length preferred by distributors. The smallest diameter is usually half inch, which is the easiest to form.
Picture above is a 3d CAD model of full 100 ft roll of 1/2" black plastic tubing. (Water content would amount to about 1.57 gallons and weigh under 13 lbs). Multiple sets of tubes can be combined to provide larger capacity.
Nice flower patterns are also fairly easy to shape up wire together (or weave together). Flower shapes might add decorative flare.
Above picture models one inch black plastic tubing. Three dimensional shaping of flat spiraling coils can integrate function and form. Two functions built into single products, table? Sauna? (Heat empty tubing in the sun, covered for greater heating. Then bending soften plastic around other objects.)
Above : The 'nautilus' spiral would need more than a 100 ft. I have found 3000 ft rolls of quarter inch black plastic sold by irrigation suppliers. (I'll be trying this here). Quarter inch tubing is easy as rope to bend. Durable tying material is recommended. The simpler 'comma' shape is to suggest abstract shapes are easy.
Above picture is another example of a 100 ft, one inch diameter tubing. Tubing comes as coils. So it could be fairly easy to 'un-spring' a new roll and tie to a set of poles. The enclosure could also serve as a drier for garden greens, (parsley, mint, etc...) An empty jug on top might provide easy access as well as a smaller space for smaller drying batches. A coil with cold water will 'sweat', meaning that air is dehumidified, within the (slowly drafted) enclosure.
Where codes permit, a more elaborate system can be built in to a home. Here in rural Maine, common sense systems are allowed in an existing home. Similar systems have actually benefited countless individuals worldwide. This page covers my efforts during the 1980s. Public health and safety is important. An existing, officially approved, waste water system and water source, are assumed. (One must comply with their own local authorities before any installation of permanent construction). Here is one more link to an article i wrote in the 1990s.
The picture above shows 2 thermostats, while the previous picture showed one thermostat. Thermostats can provide precise ranges of temperatures. One really has to determine the budget and preferred temperature range with care-- Based upon the size of the collectors, their placement and effectiveness. Also, the desired amounts of hot water production factor in. Engineering itself deserves a lot of information input, (before a fully satisfying combination is determined). Dual thermostats are especially desirable when heat collection is very large and system automation is essential. I can attest, however, that for lack of budgeting, my own system has used no thermostats water ever-- Instead, a simple on-off valve is opened around 10AM and shut just before dusk-- Naturally, the schedule is sometimes forgotten, with cool water infiltrating-- Such luke warm water has typically been used for clothes washing, to get rid of it without waste.
Note: These pages are placed in the public domain and are furnished "as is". The author assumes no responsibility for the use or misuse of the concepts in this series. All authorities should be satisfied first, as might be required, by relevant laws, before any building proceeds.