Perhaps the person booking your cruise added the gratuities on for the convenience of having them pre-paid.
Many ports are easy to do on your own. Perhaps they might have a hop on/hop off bus. That would help save some money on excursions. Other more complicated ports you may want to stick with a ships tour. Do some research on your different ports. You can check our Port section on this
site. Set your per-day budget and try to stick with it. Remember though, it is your honeymoon and you do want some special memories.