Re: What is this and where can I get another one?
Lots of electronics equipment, microwaves, etc. uses such for feet. Just go spend an hour or so at a thrift store. They are fastened with a screw. But use a self tapping stainless screw for your boat..
Lowes or most hardware stores should carry them for less than a dollar.
Suggestion: after fastening, fill it with silicon rubber to keep the water out.