How do I increase space between deck boards?

Just sweep it off, decks are built to code and thats not something you should mess with unless you know all of your local codes as they vary from state to state

this should have been done at the time the deck was laid, but you could get a pattern going by stenciling a patter you like and using a wood chisel and hammer you could variegate the pattern across the deck.

If this is a typical new wooden deck the boards need time to dry and shrink. If it's at least a year old then you really should unscrew the boards and re-space them.

I have limited experience with decking materials- I've helped build decks using cedar decking and treated fir decking. These hold up best when there is spacing for drainage.

I'm not familiar with deck products that have boards butted close like you're describing so I'm assuming it's been installed wrong.

Drilling or chiseling drainage holes might be better than nothing but isn't a great solution.

Sounds like it was not built to code - all decks are supposed to have spaces between them to allow water to drain. You could drill some holes in the boards.

Maybe get a couple shims n hammer them in all the cracks? Maybe they will budge after a rain or two? .. or could try a radial saw n very carefully cut between the boards making sure to not hit n e nails n only set the blade as deep as the boards as to not damage the supports underneath! Then sand it, clean all dust with hose, let dry completely then re-stain or paint?