How to get summon boat in heroes of might and magic 4 in to the jungle campain

After you have visited all 6 oracles, it will reveal the center of the map and you must defeat two sets of four dragons. I don't have the exact coordinates, but I had two heroes digging around south of that area.
It is just below (south) of the lava area. If you have a few heroes digging around it shouldn't take very long. Much easier if you have defeated all of the AI first.

You are looking for a Scroll of summon ships. Once you get that, you can take it to the waters edge, summon the ship and cross the water. Go to the structure on the island and the scenario will end.