Congrats!
I would recommend that you put on your invitation something like "Join us for a port side ceremony on CCL Miracle on XX day at 10:30 am" On my save the dates I mentioned a shore side wedding in Nassau Bahamas since had not confirmed the time of the wedding. I also attached the cruise information to include the travel agents info.
I opted out in hiring TWE due to the reviews I have read...I was able to find a great WC in the Bahamas and saved a lot of money. Also, if you are looking for a good travel agent, checkout Liberty Travel, I got a great rate for my guest for a weekend cruise. They also give you cabin credits to use on board the ship for you guest! Also if you get a group rate for you guest early enough, they can pay through a period of time vs. paying everything at once. I was able to give my guest up to six months to pay for the cruise.
Good Luck!
Ceci