There is a product made that goes under the decking. That allows you to put a nice ceiling on the underside, add lighting, etc. I don't know what the product was named but a quick search should pull it up.
'97 Prostar 190 - LT-1
Prior boats - 2009 X14, 2008 X14, 2005 197, 1988 Tristar 190, 1989 Prostar.