Curse users are immortal, this means they have an infinite amount of time to train and become resilient to the pollution from Durza’s attack. Furthermore, this resistance should be innate in a curse user. If a normal person can swim in the ocean without any problem, this means that there is an inconsistency with the “magic energies clashing” claim, and it can be discarded. Now that this has been removed due to its illogical roots, we can now run every scenario mentioned minus the “magic energies clashing” nonsense.
1: Normal people can swim in seawater
2: Curse users can swim in seawater
3: Wotan can create usable boats out of his curse
4: Cursebeard can possess more than 1 curse
All of these are things which can be seen in the game, meaning that for all things considered, the “magic energies clashing” is nothing but utter nonsense and can be discarded from the story entirely. Up until now, to my knowledge, curses were called curses because they sentence you to live an immortal life, the curse being that you will have to see everyone else around you eventually die, unless they too become curse users. Moreover, this is not a novel concept in AO lore, there are plenty of things which grant something great like immortality but curse you with something else.