This exercise works a whole range of muscles including quadriceps, hamstrings, abdominals and lower back.
Strengthen your abdominals and back while sitting in your chair.
Lean back slowly, tighten your abdominals, and then sit up tall again.
However, if you are not self-conscious about your abdominals and are simply considering the tankini/bra combo to accommodate your bosom, then there are numerous other swimsuit styles that can meet your needs.
As you strengthen your abdominals with boat pose, you'll also want to strengthen your back.