I often use scuba gear to map out the area around a client's pier, and I almost always find something that they didn't know was there (at least that is what they say). Old appliances - refridgerators, microwaves, etc. - not a good idea. One clients best spot that he caught crappie from (before I put in my PVC reef) was an old toliet. He had no idea it was there - just knew that he caught fish there.
Another client's best spot was a plastic lawn chair, that he thought was stolden years ago. Apparently the wind had blown it off his pier.
We need to all try to use materials that will not harm our precious water. If I find materials that shouldn't be there - I'm going to do my best to pull them out and dispose of them properly.