I'll preface this by saying I've never tried this in a real world application myself, but I was curious and found some instructions for creating quick harnesses out of webbing from a web search.
I want to add that I am in no way endorsing this for climbing or prolonged use beyond a static hang or an emergency situation. I've heard and read that rope/webbing harnesses can be quite painful and can possibly cause injury if someone were to fall in them, due to lack of padding.
The first way I found is from the American Alpine Institute blog, which, half-way down the page mentions the diaper sling shown below, and then another alternative called the Swiss Seat shown in a Youtube video or similarly here.
The next is called a Diaper Sling. From here:
Diaper Sling
You'll need one piece of 1-inch tubular webbing. The length will vary
depending on the size of your waist and thighs but 12-feet is usually
plenty.
Create a loop out of the sling by tying the ends together with a Water
Knot.
With the loop horizontal (parallel to the ground) run the top half of
the loop across the small of your back, and the lower half of the loop
below your buttocks. Keep the knot towards the front so that it is
smooth against your back and thighs.
Then, reach between your legs, to grab the lower loop and pull it in
between your thighs. Keep holding on to the original loop!
Finally, clip the whole mess together.
Ta-da.... you have created a diaper sling.
I used the instructions to create a test diaper sling myself, and it seems like it would hold me, but if i really needed a harness I'd probably opt to spend the time making the Swiss Seat.
As I mentioned above, mountaineering harnesses can be little more than stitched webbing with some added thickness which might be a better alternative if this is going to be more than a one time use.
One last comment addressing your question. I don't know your exact use case, but using rope while crossing streams/rivers is generally risky business. For example, "The two hikers had tied themselves onto a rope that had been placed across the stream earlier this summer. They lost their footing and were pulled under by the current." Though there are techniques to use rope and anchors to successfully cross streams/rivers, when misused the rope can get pulled into the water, creating lots of drag and usually taking whoever is attached right off their feet. You might be quite experienced with this, so I'm just leaving this as a general fair warning.
