Pretty much, yeah.
Alpacka used to use wide velcro (and maybe still do on some spray decks) but people complain about water getting in through the velcro.
You can buy “waterproof” (more like water-resistant) zippers of various lengths on Amazon and elsewhere. Some of them even have a heat sealable TPU coating on one side. Remember you’ll need a separating zipper (like zippers used in jackets), not ones that are closed at the ends (like zippered pockets).
#5 zippers are a bit small. I’d prefer a #10 for a spray deck but they are less common.