Let me share what i know about accessing that diving mission since i've done it both ways. The good news is you don't need to pursue anything romantic to experience the underwater exploration

first things first, you'll need to work through judy's earlier missions - complete both sides now and ex-factor, then tackle pisces. When you get to pisces, here's something crucial to remember: don't take maiko's deal or accept her money, or you'll miss out on the diving opportunity completely. You can either take down the tyger claws or support the dolls' independence, both choices keep the diving mission available.

After wrapping up pisces, give it about a day in-game time and judy will reach out about exploring an underwater town from her past. During the actual diving mission, if you want to keep things strictly friendly, just decline staying overnight at the cottage and avoid any flirty dialogue options. Maybe interesting to note that when morning comes, you can choose "a pleasant distraction" to keep the conversation casual. You'll still get some cool diving gear memorabilia and access to judy's apartment, plus you get to learn about her background without any romantic complications. The memories and story she shares are worth experiencing regardless of whether you're interested in romance or not.