Ideas may be priceless, but without a working prototype they are also essentially valueless.
If you think the thing will work, and if you want to get any traction on your idea and make it valuable enough for someone to consider purchasing (in order to make water purifiers based on the patentable idea), then you need to demonstrate that it actually can be done, and some guidance on the production cost. You need to do it.
At this point in your product life cycle if you don’t have the requisite engineering / prototyping / manufacturing skills to make that prototype yourself, then you may be able to partner with someone who does, for a share of future earnings, or for a flat rate. No one who’s worth doing business with will work for free.
For this reason, if you plan to be an inventor, you should start to develop those skills yourself, so that you don’t have to rely upon others to do the scut work at this point – and not steal your idea for themselves.
In addition to the foregoing, if you do plan to sell your idea (hopefully patented) for anything more than nominal value you should also contact a patent attorney, who can assist with the patent research to ensure that the idea has not already been patented by others, and if not, to protect your interest in your new idea.
Definitely start to keep a bound and dated log book of sketches, proposed specifications and features, etc. Documentation is crucial to patent application and protection.