Fairy Godmother - I don't know if Randy does underwater photos. I have never seen any from him, but it would be worth asking. He is so lovely, and will be happy to answer all your queries.
I know that if you use DPS, you can shoot a TTD at a water park. The session has to be done before the park opens and you have to pay for lifeguard(s), so it and work out quite expensive. Good luck, and keep us posted of all your plans, sweetie.
Diamond Girl - I am not 100% sure whether they have been given the wrong information, as the rules change all the time!! It may depend on where they wanted to hold their TTD session. Certainly if you want to shoot at any of the parks or the water parks, they would have to use a DPS photographer.
The question about whether couples, not using DPS, are allowed to wander around the resorts in wedding attire, does come up from time to time. Officially, they are allowed, but there have been reports of people being 'discouraged' from doing so!!! You can understand why Disney would like everyone to use their photographers (and be obliged to officially 'encourage' all brides to do so),but as far as I know, the rules remain the same and allow for anybody to be at resorts, beach areas etc., in wedding attire.
Certainly, when we shot at the Poly with Randy (less than a year ago), we were there over two hours, were on the beach, at Sunset Point, in the Lagoon and in the pool and nobody bothered us once. Quite the opposite, in fact, all the cast we encountered, greeted Randy warmly and congratulated us!!!
All that said Randy is extremely well known at WDW. The cast know he will stick to the rules and not do anything to upset other brides or other guests, and so, I think, they leave him to his own devices.
If your son and his fiancee are using a photographer not so well known at WDW, they may encounter more suspicion, from cast, than we did. I know Randy has recently replied to another Disbride (a S/D bride), that he is happy to do her TTD at the Poly, so I guess he is, currently, still happy to shoot there.