Hmm... good start! If I were to do the same, I would get it commercially done, but making this by hand can be interesting. In this case, I would do the following:
1) Use a small box that resembles a treasure chest or wooden crate, etc. the bottle can fit in.
2) Decorate it by stuffing small amount of easter grass on bottom. On the inside of the opening flap, paste an oval shaped colored foil or glossy greeting briefly stating "You are cordially invited to...". May want a palm tree or 2 to pop up when box is opened.
3) On top of the box, also make a mysterious request to open the box.
4) Dye the bottle blue with varying shades to simulate waves of water.
5) Have the invitation done on heavy paper that can easily be rolled.
6) You may have an old tearing effect done on the edges of the invitation, or lightly burn the edges with a lighter.
7) Roll the invitation, secure with a ribbon, and place in bottle with at least a quarter of it sticking out so it can easily be grasped.
8) Set palm trees for pop up effect (optional), place bottle on grass, and seal box.
Hope this helps! Congratulations on your big day!