From her apparent interest, I get the feeling that what is entailed to dive just may not seem appealing. (Its unfortunate when one interest is not shared but I bet there are things she enjoys that you cannot imagine why any one would.)
But feel free to offer again, perhaps after hearing of your adventures definitely continue to dive with out her she may reconsider. If she lives on the lake Id say you have an excellent chance of success, and from instructing (other sports) I have a suggestion.
Pick a good time and bring up the subject again, in the gentlest way you can, practice tone and wording. Im thinking the vibe she receives from you is; offering the opportunity, thinking she would really want to experience and share the good things scuba diving provides. Make no indication that; its easy, all you got to do is
or I just know
anything about she feels or thinks. If that is well received then offer the heated pool as you suggested and private one on one, instruction.
You sound like a caring and considerate partner but shes already had a bad experience, perhaps in more ways than you realize. I think it would be important to go out of your way and make what may seem ridiculous over kill steps to insure she has a good experience. Avoid letting her know to what great lengths you have gone for her until you are trying to CYA about some unrelated faux pas.
Approach LDSs or private instructors with the situation; do they have someone they think would fit the bill, possibly experience with children. With young childrens short attention span you have to have a lot of tricks up your sleeve.
Then, interview the instructors. If possible find maybe three that are intrigued by the challenge, willing to adapt and try a different approach. Avoid someone that encourages you not to bother because its just not for everyone.
Insist well suited, meaning variable size and configuration, high quality equipment is available and will be used. You, she and the instructor may think, say a minimal BC would be less restrictive, but then perhaps a vest with no sternum strap might turn out to feel better. This may mean going way out of your way to rent or borrow. Insist the mask and snorkel are comfortable, do not let it slide because she has already demonstrated comfort snorkeling. You want to have everything comfy to focus on the diving aspects, not a distraction adding to the uncomfortable new skills. Take every opportunity for her to be relaxed and not assume that she should be.
Then have her meet the instructors and choose who shed like to have a go at it again with. Remember she will know best who feels good to her.
Ask if she would like you to accompany her to the lesson or not and respect her choice with no discussion unless she initiates it. Encourage her to immediately voice her feelings and concerns to the instructor. Stay neutral if she wants to tell you about them, you discussed them before and didnt get anywhere, listen but let the professional deal with them.
Then have no part whatsoever in the lesson. Nada, not observing, not even thinking she wouldnt know if you did. There are, perhaps subtle and unintentional, pressures evoked by your presence, eliminate them entirely.
Be open to discussion afterwards but do not insist, let her lead.
If discussing; be prepared for her to babble about every little thing they did.
Avoid asking questions, instead repeat what she says and praise everything that she is pleased with. If youre feeling a bit silly, youre probably doing it right and shell let you know when you can stop.
Be prepared for the bulk of the lesson seeming to involve speaking with little actual skill work and ready to provide more time or lessons.
Be prepared for her to be in tears, angry or disappointed and perhaps at another time, ask if she wants to try a different instructor. Be prepared for her never wanting to do that again.
Be prepared for her wanting to show you what she can do, and how easy it is.
Be prepared to reward success with a treat, preferably something you know she likes and is some sort of a sacrifice for you to give.
Be prepared for her to want to share the wonderful experience of Scuba Diving, real soon.
Be prepared for her to dive the pants off you in no time.