I'm non Korean, but I've made Napa cabbage kimchi a couple of times following instructions from the internet0]. The seafood is optional. I used fish sauce, but that's it. It came out fantastic, and I encourage you to try it! Her video that accompanies the recipe is worth watching too.
I've made vegetarian kimchi dozens of times and you can get away without it. Plenty of purists have every right to tear me apart here. But as a vegetarian, between 1. giving up one's principles, 2. having no kimchi or 3, staying true to one's principles and still having kimchi that's 85% as good, I'll take the latter, thank you very much.
If you're not veg you can use prepared seafoods - cured versions, sauces, etc to get most of the way there. As a vegetarian I use soybean paste to sub in for the umami flavor and some of the salt / fermentable sugars I'm otherwise missing.
The only seafood you really have to use (and if you are making vegan then even that is not needed but the taste will suffer) is those special anchovies which are frozen and salted so I'm not worried (making kimchi myself from time to time). The only really important thing is that the result has enough salt in it, but recipes make sure to drive this point home.
