Original portal about 800 blocks from base in overworld.  Boated back to base, created a new portal, which linked up to original portal in nether.  Calculated portal placement and created new portal in nether, which created a new portal in overworld, about 150 blocks from base, in a ravine under swamp/ocean.  Broke new portal and made my way back to base.  Jumped through base portal again, which led to second portal in nether, but going back to overworld keeps leading back to same ravine.  Each time, I'm breaking the portal, removing an obsidian block, creating a graveyard of broken portals under the ocean.