Recently, I asked whether or not the other plant groups besides the angiosperms could evolve to adapt to being submerged under salty ocean water 24/7. While marine conifers, ginkgoes, cycads, ferns, horsetails, lycopods, liverworts and mosses are not possible on Earth, in an alternate Earth, they could have the opportunity.
So in an alternate Earth, the world's shallow waters are green with forests as complex as our kelp forests, if not more so. But plants being plants, their diversity raises some concerns. In marine jungles like this, what sorts of changes would the conifers, ginkgoes, cycads, ferns, horestails, lycopods, liverworts and mosses need to adapt to life in salty ocean water?