Beach semi-formal

[Rex Tyranas]

So I have a family wedding later this year which will be in Aurba of all places. The invite specifies the dress code as "beach semi-formal" and all I know about this is that I don't have it. For those of a more sartorial persuasion do any suggestions as to what this would look like and any good places to source said garments?

I'm hoping the collective wisdom will mean I don't look like a complete berk...

[Rex Tyranas]

Thanks for all the useful or entertaining suggestions. My wife is American so this is her side of the family so I imagine it will be a bit preppy. Semi-formal to me sounds like a mullet, the compromise between "business" and "party" ends up with something that is neither.

What are people's thoughts on boat shoes rather than flip/flops?