Since there is no way, in any possible circular boat course, for a boat to go up even the slightest water slope (as described in MC-111270 which, although closed for being a duplicate of boats being underwater (MC-89944), is actually quite different even if the cause is the same), my boat course was forced to use a nether portal to go up, however, when the player in the boat goes into the nether portal, the player in the boat does not enter the nether world.  Bug also happens when player is in the nether world and tries to return to overworld.