Deck Prisms! (Patreon)

So there were a lot of good guesses about what these holes in the deck are for..

some of my favourites were; 

Periscopes, Mug holders, To let the water in (!), Deck-Head (!?!), Beer elevator, Tomahawk missile ports.... 

but of course the correct answer, which many people guessed, was Deck Prisms (sometimes known as Deck Lights). These glass pieces distribute sunlight into the living spaces really effectively thanks to their shape, and don't take up much space on deck. I planned their locations before we laid the deck, so the place where they cut a deck plank fits in with the overall butt layout for the deck (and actually we left an uncorked joint in those places so we could use shorter planks).