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I've forgotten a lot already. I remember that you need to explore more tunnels, visit the Seers' huts, and somewhere there, a cartographer of the seas should be located. It's a shame I forgot... But I can try to go through it again! I'm ready!
Added after 19 hours and 21 minutes
Well, I made it to the cartographer! Now I'll describe everything in detail. So, when you start the mission, pick up the chest and wreckage, and immediately head to the first island. There's a wooden Tower (they also clear the fog) and a Mage's hut (they show distant locations). Sail to the island on the right; there's an entrance to a tunnel and a Tower. Go into the tunnel; if you go to the right, you'll end up on an island with a shipyard and a Mage's hut. It's better not to turn right but to go straight; you'll end up on an island with creature dwellings (Cerberuses also join you there). Hire them and sail to the island with a Hill Fort (to upgrade creatures), and then sail or go through the tunnel to the island with a shipyard. Kill the unicorns and visit the Mage's hut. Then, sail to the island with Ents, kill them, and pick up the Cave Demons. Next, sail to the island that is above and to the right of the island with creature dwellings. There's an entrance to a tunnel on it, but it's better not to go into it; instead, explore the island. You'll see dragons; kill them, then go further, and you'll see the cartographer with a ship in the sea, and to the right, there's an altar of the first level. Learn the "Summon Ship" spell there. Stand knee-deep in the sea and summon it, and then go to the cartographer. I think you'll be fine with him. If you have any questions, ask!