Royal Queensland Yacht Squadron is adjacent to Manly Harbour Marina and has a great value Hot & Cold Seafood Platter for 2 available lunch and dinner on Valentine's Day for only $90. Facebook has more details read my review.
Keith's on Cambridge has wonderful Marina views and is another perfect romantic setting for a dusk time Valentine's evening meal. Breakfast, Lunch or a cake and coffee are also options. Check their website for more details and read my food review.
Manly Harbour Village is full of eateries for a perfect Valentine's Day meal with the Manly Deck providing excellent views and the recently opened Shucks Oyster bar.