In church: Suit or the equivalent (T1/P1 for military personnel in uniform).
On board: Suit, however with practical adjustments considering weather conditions - it might get chillier in the evening (changing facilities available on board).
For military personnel the dress code will be best uniform (mess dress I) or civilian outfit. However, civilian outfit is recommended on board for practical reasons.

Please avoid stiletto heels and umbrellas!

In case of bad weather conditions the party will be moved below deck.