。
A yacht you hardly ever ride is just a money pit.
Literal
Rarely ride-not yacht [topic-なんて] just [possessive-の] money-eating bug [copula-だ].
金食い虫 ('money-eating bug') is a vivid compound for something that consumes money without return — a money pit. めったに~ない ('rarely, hardly ever') is the negative frequency pattern. なんて adds dismissive emphasis — 'a yacht, of all things.' The sentence is a pragmatic complaint about wasteful luxury.