So, the TVTropes page states him to be Ambiguously Bi, as he is confirmed to have a female love interest (Squilvia) but not confirmed to have any male love interests explicitly stated. However, though I haven’t seen the Sandy Cheeks movie, there is an instance, apparently, of him finding Sandy’s brother Randy so attractive that he faints. A Sexypedia article claims that as of “Saving Bikini Bottom: A Sandy Cheeks Movie, Squidward is canonically bisexual, as he is seen openly simping for fellow Sexyman Randy Cheeks. Squidward’s bisexuality should not come as a surprise to most fans of the show, or anyone who has seen certain theories regarding him.”

In Squisery from Kamp Koral, there is the following scene of Patrick and Squidward seemingly doing romantic things together, like Patrick making Squidward soup, Squidward imagining Patrick as a blonde muscular guy while idolizing him and looking amazed and seemingly falling in love with him.

Plus, he is widely headcanoned to be bi.