I do not think the Psijic Ruins get closed off at any time, as you can always leave the house through them. That beach is accessed via the Ruins to get to the Relic Vaults at the other end of the path, and that door may get locked before and after the quest. The northern end should stay open.
You can't swim to it. I accessed it through the library during the Summerset zone questline, but now the door is locked. I was wondering if there was another route. Is it possible to get access after finishing the quest? I have finished the quest.