From ACSB about 8% of biology PhDs land in a tenure-track faculty position. Numbers are based on workforce numbers in the biomedical sciences. In marine biology there appears to be no lack of interested students entering the field, but I'll wager even fewer marine biology graduate students stay in science or research for their career.
As a student or postdoc I think it's always a good idea to explore as many career path opportunities as possible. You may find a career outside of academia that suits you better.