can I use a thick coating to fill the gaps between planks on wood deck?
I have installed membranes underneath decks and ran them to gutters to provide an under deck patio space.  To answer your question, no you can't do that - for the easiest reason - because it won't work and your guests will get wet.  And for a hard reason, it will tear up your deck with wood expanding and warping due to pressure or wood rotting due to standing water.

I would ask a follow up question that asks possible ways to divert water, so that people can answer that.