Hi Ambivalent
I will not suggest Similan Islands or Phi Phi Islands, especially when you are a novice diver and have a non-diver with you. If you like to have a relaxing day for both of you and get some more experience as a diver then book a trip to Racha Noi/Racha Yai with three dives. Most dive sites around these islands are suitable for all levels of divers and are great for snorkelers.
If you choose the Similan Islands you will be in for a very long day. Pick-up is around 6.30, your first dive around 11.00 and you will be back home around 19.00. Although the diving is great, I think to much time is spent/wasted on transfer. The Phi Phi Islands dive trips are usually combined with a a dive around Shark Point, Koh Doc Mai or the King Cruiser Wreck and with your experience, I would advise against diving any of them (King Cruiser is a no-go for unexperienced and beginner divers). Hopefully you'll have many years of diving in front of you, but improving your dive skills is cruisial for great dive expeience.
This is my answer to another diver just a few days ago; "The dive sites at Racha Yai are suitable for all levels of scuba divers. At Racha Noi, most are suitable but some are not (south point for beginners is a big no-go) Visibility is normally in the range 10 - 30 meter and currents are from nothing to mild but some days around full moon a bit harder. Last year we spotted whale sharks and manta rays, although they have become more and more rare. You'll find all kinds of tropical fish and corals so get a really good dive guide. There are several day trips offering 2 dive at Racha Yai only and they are usually very crowded (not recommended for a relaxing day out)
The prices are slightly different and ranging from 3,500 THB - 4,100 THB. Equipment rental will be 400 THB - 800 THB for a complete set. Boats are leaving the pier around 8.30 - 9.00 and will be back around 17.00. The prices include pick-up from your hotel (some areas have an extra charge), breakfast, lunch, fruit, water, coffee, tea and three dives with a dive guide."
I hope this helped you a bit. I'm based in Phuket so feel free to contact me if you need some help and more advises.
Michael