pics

For starters, I've been writing about marine crustaceans on and off for some years, and for raw taxonomical information, the best resource to me, bar none, is the World Register of Marine Species (WoRMS). This is going to use official taxonomical names and classifications, so that's best-reserved as a place to learn where an organism is within the tree of life as we understand it today, but the nitty-gritty specifics of where the organism falls might not make sense to you if you're not familiar with that specific area. I don't really recommend going into it hoping to learn general facts about marine biology en masse, and most of that information will be on places like Wikipedia anyway (WoRMS is just sometimes more up-to-date). Wikipedia will tend to be more user-friendly with broader information; the infobox at the top of each taxonomic article gives a placement within the tree of life.

At the bottom of that plankton article (desktop version) is what's called a navbox that has relevant links to articles about different types of plankton.

If you want to check out cool pictures of specific groups, I recommend iNaturalist and Wikimedia Commons (iNaturalist especially, as much of Commons' content is from there and it's generally well-reviewed for accuracy).

Those are three general resources, but it's hard to point in a specific direction without a more specific goal.