Yep, you're going to have to just get some water plants started in that pond. Hmm, got a little boat? You need to plant stuff like water lilies in about a foot or two of water so looks like around the island is the place to plant. Marginals could grow on the little island, too. It's way too big to try and clear it up with filters and gizmos. If you don't get some fish in there soon, it will be a mosquito habitat as soon as it warms up, too. Fish need air in the water and algae to eat until the mosquito larvae turn up. So, aerate first, plant second - lots of plants! Then get some fish in there.
Once you get a balance established, you'll be amazed how it will clean and clear up, but you need a fair sized aerator for a pond that big.
See if there's anyone locally who has a pond - often they will have plant starts you can trade for or they'll even give away starts if their ponds or water gardens are overgrown.
You could grow the big native lotus Nelumbo lutea in that pond - they're beautiful big, yellow fragrant flowers and huge leaves 2ft across, and they're a native plant, hardy to zone 4a.
