Under 17 USC 106
the owner of copyright under this title has the exclusive rights to do
and to authorize any of the following:
(1) to reproduce the copyrighted work in copies or phonorecords;
(2) to prepare derivative works based upon the copyrighted work
Since Owner has that exclusive right, only he may do or authorize the creation of a copy or derivative work. As defined in 17 USC 101,
A “derivative work” is a work based upon one or more preexisting
works, such as a translation, musical arrangement, dramatization,
fictionalization, motion picture version, sound recording, art
reproduction, abridgment, condensation, or any other form in which a
work may be recast, transformed, or adapted. A work consisting of
editorial revisions, annotations, elaborations, or other modifications
which, as a whole, represent an original work of authorship, is a
In editing out the watermark, you would be creating a derivative work.
It is not actually required that you pay for the work or credit the owner of copyright, all that is required is that you have his permission. If he allows you to make the work, removing watermarks and not paying him, then it is legal. If he does not respond affirmatively (no answer, you can't find him, or he denies the request), you do not have permission. The fact that it is physically possible to make a copy does not change the legal situation. Owner can then sue infringer, where the amount recovered depends on the circumstances (what would the licensing royalties have been; how willful was the infringement, etc.).
In the US, there are annually about 1,775 thefts per 100,000 people, yet theft is a crime. Not all thefts are actually punished, for many reasons. Copyright infringement is particularly popular because it is extremely hard to control making of copies, and because many people either don't know or willfully disregard copyright law. There was a meme, some decades ago, to the effect that if you make a little change in a work, then you aren't making an exact copy (but copyright law doesn't just prohibit exact copying). Most copyright owners don't have the time necessary to search the internet for pirated versions of their works. With DMCA take-down, it has become somewhat simpler to put an end to specific instances of infringement.