You mentioned ASMP. Did you join as a member and put your information in the Find an Assistant database? If an ASMP photographer wants to hire an assistant that they aren't already connected with, that's where they are going to look.
Same applies to APA. These are paid professional membership organizations. Networking happens within the organization. If everyone goes outside of the organization, the the benefits of membership are reduced.
Wonderful Machine reps photographers. Many of those photographers are members of ASMP, APA, and other professional organizations. It is not to their benefit to respond to solicitation emails from you, much less to hire you instead of hiring someone from within their professional organizations.
Watermarks didn’t stop anything. People used Photoshop to remove them or cropped them out of the picture. Some people didn’t care about the watermark at all and used the picture with it visible.