Can the questioner be reunited in Paradise with someone with whom she had an illicit relationship, and can the deceased intercede for her, knowing that he died by drowning?

What happened was zina (unlawful sexual intercourse), and the saying "I marry myself to you" does not change the fact that it was an invalid marriage, without a wali (guardian) or witnesses. It is a crime for which Allah has promised torment to its perpetrator, unless there is sincere repentance.

Repentance must involve ceasing the sin, regretting it, and resolving not to return to it, along with concealment (of the sin) and increasing good deeds. Part of sincere repentance is that the heart should not turn back to the sin, nor should it remain attached to the person with whom the transgression occurred; rather, it should despise it. The greatest hope should be Allah's acceptance of her repentance.

Your concern should not be about the possibility of reuniting with this man in the Hereafter; a woman in Paradise will be with her last husband from this world. Intercession has conditions. Relationships built on anything other than piety in this world will turn into enmity on the Day of Resurrection.

On the Day of Resurrection, every person will be preoccupied with their own destiny. You must turn to your Lord, learn matters of your religion, and frequently seek forgiveness and supplicate for the acceptance of your repentance and for a righteous marriage.